The most memorable death in any of my games befell an NPC - Shaaladel, the Emperor of Light, Lord of the Elvish People.
Shaaladel was the last survivor of a group of about eight people who were vying to take control of the world. The other emperors had been stronger or had larger armies, but Shaaladel was a canny politician, and he had played his foes against each other. He was also completely despised by the party.
Way back when the party was only 6th level, they were investigating the castle where the recently-murdered Emperor of the World - Draco Coaltongue - had died, when an army led by Shaaladel arrived to 'investigate' the same thing. In truth, Shaaladel had arranged the assassination, and now he was seeking to locate an artifact his assassins had stolen. There was strange magic affecting the world so that if anyone teleported they would be burnt to a crisp, and only the Elves seemed to possess magic that would let them surviving the effect known as The Burning Sky. The party wanted to know why.
Hiding in the castle, the party was gathering information and deciphering clues, but they could not escape Shaaladel. He sent his daughter Shalosha, a skilled warrior and spellcaster, to track them down, and they were captured. But they were not executed, because Shaaladel always preferred allies to dead enemies. He intended to leave them alive as prisoners, but one PC tried to make a prison break, and was killed. This spurred the rest of the group to break their bonds and attack Shaaladel, but they had no chance against a magically-defended emperor and his army.
They were captured again, with ease, and left to be tortured for information while Shaaladel went to retrieve the stolen artifact. He left his daughter Shalosha to keep watch on them, though she was never present during the torture. With the aid of unexpected allies, the party was freed, and they took Shalosha as a captive, then chased after Shaaladel. Various misadventures followed were the party thwarted the emperor's plans, and the emperor struck back at them, but as the months passed, Shalosha was able to see her father from the party's perspective, and she realized he was a cruel, prideful man who was no better than any of the others seeking power. Instead, Shalosha begins to fall in love with Rantle, a human PC who is a stirring leader as well.
With the aid of Shalosha and other allies who hate Shaaladel, the party defeats the other emperors, or pits them against each other so they can stay out of the fight. Then finally, only Shaaladel is left. The party is powerful now, possessing the artifact Shaaladel himself had once sought. Shalosha possesses the Elf magic that lets her teleport the party safely (it just requires a few minutes to cast a super-powered 'protection from fire' spell first), so the group goes to confront him. But recently Shaaladel had tried to befriend them as allies, so the party is uncertain if they even need to fight.
Until, that is, the bastard betrays them and tries to kill them. Shalosha is kidnapped, and the group teleports after immediately, not wanting to spare the time to cast the safe teleport spell. They arrive for the final showdown in a burst of flame outside an ancient temple of secrets, just as Shaaladel is coming out to address his captured daughter.
Shaaladel sees his enemies, and knows the time for negotiating is over. He orders his soldiers to attack, and he himself charges into the midst of the party, whirlwind attacking, cleaving, nearly cutting down Rantle, the man who would wed his daughter. Then Fayne, a PC elfwoman whose family had been killed by Shaaladel's armies, grapples Shaaladel, keeping him from using his brilliant swordfighting skill. The party is holding off the other soldiers, and though Shaaladel struggles and manages to slit Fayne's throat, she holds onto him, trying to force her blade into his chest.
At this moment, Shalosha's will breaks. She hates her father, but she also loves him and cannot let him die. She forces her way through the melee and grabs her father, then casts teleport to escape with him. Shalosha has fire protection, but in her desperation to rescue her father, she forgets that he doesn't.
There is a blinding flash of fire, and they reappear in a beautiful forest grove, Shalosha embracing the incinerated body of her father. She cries out and releases him, and he falls to the ground, dead by the hand of his own daughter.
The group decided the death was very appropriate. They all had a reason to want to kill Shaaladel, and it wouldn't have been fair if just one PC had done it. But the poetic irony of Shaaladel's demise appealed to them.