My 3 favourite characters, are both from dnd 3.5 forgotten realms.
First is, a human sorcerer who has 2 opposite personalities, chaotic good and chaotic evil. His power came from this evil spirit, so casting spells, and growing stronger in magic increased the chances of his evil side taking over his body. Because of that, he was very diplomatic, peaceful, and he never casted a spell unles it was really, really necessary. When the evil one took control, he tried to bring them into situations where using magic would be necessary for survival, or he put his friends with his plots at risk, thereby forcing use of magic, even if the good one managed to regain control. His good side searched for a way to get rid of this cruel side of him, even if it cost him his magical powers. Eventually, the sorcerer grew stronger, and before the good side managed to succeed, the evil one plotted to take control forever, by transforming to a lich. Durin the ritual, sorcerer kills himself and reanimates in his old "body" with powers of a lich. Using this, the evil side killed the body, thereby "killing" them both, but only made himself a lich, removing the good side forever. Of course, campaign wasn't revolved around this sorcerer, but it was his motivations and actions which manipulated the entire party and his good side to danger. So yes, my character was the stories villain. And the campaign's end? Well i disguised myself, and continiued to act like the old me, mostly good, peaceful, but sometimes evil. But well, i just couldn't hide being a lich for too long. In a combat, i died. Of course, i came back in my phlactery, but they didn't know it, and they just thought me as gone. My ultimate goal was to become a demilich, totally ruling the magic in Faerun. The purpose was, to teach magic to as many people as possible. Demiliches are immune to magic, so the more wizards there are in Faerun, less people to oppose me.
In the shadows, i manipulated the party to even more mess, all to take out strong wizards with strong positions in big cities, like archmages of wizard guilds and such.
But, after a miscalculated plan, they found out it was me pulling the strings. Their reactions were SUPERB

Of course, they came after me, we had a loong talk. They weren't necessarly good. There was a human female lawful-neutral monk who was in love with me. She was shocked to see me lying that much to her. Well, I was in love
with her too, both good and evil me, but the evil me saw it as a weakness and tried to deny it. She closed herself to the monastery she trained in, trying to focus on her path to perfection and forget about me. The rest of the party, a barbarian who owes his life to me, and a fallen-paladin (it was because of me he became fallen). They came at me with much anger. Of course, i had a trap waiting for them, but before attacking, i tried to talk my way out of it, to see if i can gain new "minions". The paladin was on the edge of becoming a blackguard, and the barbarian really thought of me as his best friend, he agreed to side with me if I let him punch me in the face.
Without the monk i love around, distant from her for a long time, i just had to suck it up and admit that i love her, and that in order to prevent those emotions getting in my way i had to keep her close, instead of distant. I went to the monastery, and just removed my disguise in the front gate, where she could see mee. She told me that she needs to remove her emotions in order to achive "perfectness", but i convinced her to come, saying that she needs the eternal life i'll give her to achieve that perfectness, and told her that "we" humans need emotions to fill our weaknesses, that no human is perfect alone, and perfect can be only achieved together.
In short words, a sorcerer with 2 personalities who became a demilich who's insane with power, enjoys manipulation and lies and destruction, but also in love with a lawful evil vampire monk. (There's no such thing as "evil" can't love or "good" can't hate. Every sould has both emotions. Not to start a "good" "evil" debate, but i wrote this note in case someone says "evil fell in love?". If you aren't convinced, you can go read Rich Burlew's posts about evil knowing friendship and love.)
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Second is a human, wizard this time. He had a very fragile body, he was physically very weak, and had a very bad memory. His family died while he was young. But his father already sent him to a wizard school, because the boy was very intelligent. But thanks to his bad memory, he couldn't to well in school and got kicked out. However, he was thirsty. Thirsty of knowledge, of magic and of books. He found a job as a librarian, and read all the time. He slept as much as elves, but this made his body even weaker, although he needed this kind of "sacrifice" to make up for his memory.
He was a genius, he was able to cast all spells better than everyone, he had a special talent, but sometimes he forgot the words of the spell he is casting, at the start, or at the middle

Combats with this character was really funny

His ultimate goal was to know everything and live a peaceful life, but as the campaign progressed, he got looked down and ridiculed, always disliked by everyone, thanks to his bad memory, fragile body and unstable mind. He was really childish, as insane as you can be. He didn't want to hurt anyone, he wasn't cruel. But events made him rebel against his way of living. He suddenly gone rampage and yelled "ENOUGH!" when he got a beating in a tavern. He killed everyone there, burned the building, and he was about to animate the corpses of people who looked down on him, to have the pleasure of killing them again. That's when the party interfered and stopped him. He acted like he had regret, but there was no turning back, his ultimate goal was now killing everyone who looked down on him in past, being powerful than all, and being unchallanged. The party was no different to him...After they fought a dragon, he took everyone being wounded as an opportunity and attacked them, killing 2 of 3 party members, but the paladin managed to put an end to his rage. That was really a funny game
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Third is a human fighter(again human, lol). The world we played in was dominated by a powerful lich wizard. There was only 1 city not under his control. We were playing as members of those "rebels". It was a damn long rebellion. It lasted from level 1 until level 20. The Party found a scythe, an artifact forged by Nerull himself, with purpose of taking lives. This scythe was Lawful Evil, and it was bloodthirsty. It needed to cut living flesh and absorb their blood. Absorbing blood made it powerful, and after a lot of blood-drain, it was able to unleash total destruction and death. But if left unfed for around a week, it would compell the wielder to just kill something. And it was damn hard resisting its control. It was clearly an evil artifact, but the I managed to convince the party to accept me wielding it, saying we needed it's powers to take the lich out. Of course, after a time, i figured it's thirst, and tried to take as many lives as possible to prevent the scythe from dominating me. My ultimate goal was to destroy the lich, and i was ready to bear any consequences for that cause. Sometimes, the scythe compelled me to attack, but it was smart, and didn't make me kill any of my friends because their powers were needed to take more lives. It just made me charge into nearby animal, or giant, or dragon

Eventually, i was corrupt by it's power. More i kill, stronger i become, and with more power i can kill more and so on...
One time, we were working with some storm giants to kill a dragon which was working for the lich. There was also a mage aiding the dragon. That mage put a Wall of Force between the party, splitting it in 2 groups. After the dragon fight, Wall of Force was still there. I was alone with 2 wounded storm giants in a secluded place, wielding a bloodthirsty powerful artifact. Dragon fight required me to use artifact's powers, and it was thirsty, even after taking the dragon's life. I killed those giants, and party was in shock... I told them that the scythe forced me to do this. It wasn't a lie. I just couldn't see the difference between being controlled by it to kill, or killing for more power. They knew i was corrupt, but they knew we needed this scythe.
Eventually, we fought the lich. Scythe really troubled him, and almost took him out. But he decided to focus on me, and killed me. The scythe was full with blood, and it created me as a vampire in the temple we found him. The party killed the lich. As they were going to destroy his phlactery, i've gone to the capital, and killed the high-priest and rebel commander, also slaughtering the entire rebel army. I wasn't just a dumb fighter. I've hit the temple first, because they would be the greatest threat to a vampire. And well, regular soldiers are just no match to an epic-level fighter with a greater artifact scythe, even if they are outnumbering.
Then, I withdrew to the temple, and the party destroyed the lich and freed the country forever. But when they returned to the city to celebrate, they were just shocked with the genocide. They found out that the artifact completely took over me. They came to the temple, there was a very short, but challenging combat.
Well, it was short. Because i landed a critical hit to the cleric of pelor and the bard, instantly splitting them to half. The halfling sorcerer managed to disintegrate me with his rod of maximize, then, he had the other 2 resurrected. They have sealed the artifact, and resurrected me. Meanwhile, our bard has become the country's king.(After the rebel leader was brought back to life, he suggested our bard to take the lead, because he believed himself unfit to be a peace-time ruler.) He was known everywhere and loved by anyone, and his knowledge of manners and diplomacy just made him a good king. The High Priest didn't come back to life, because he fulfilled his duty and earned the right to rest in afterlife. Our cleric took the duty of the high priest. I was promoted to the commander of army and right hand of king. But well, in truth, a bard, who is chaotic by nature, is unfit to be a king. He got bored, and just disappeared, with the waitress he loved. And of course, the crazy halfling sorcerer were nowhere to be found. I took the crown, because i knew the bard wasn't going to come back, and the campaign finished with a good end. (Unlike the first 2 i wrote, lol)