What's the best you've felt upon killing a monster / npc?

[Yqatuba]

By this is don't mean in the "yay! I got a lot of xp/loot" but more in the "take that you bastard!" sense i.e they were so evil and despicable that killing them felt great.

[Gorbacz]

Killing evil monsters/NPCs brings me little to joy. Murdering orphanages full of kids, on the other hand... Those screams ... they soothe me like a lullaby.

[Val'bryn2]

Lich Queen of the Githyanki in the 3.5 Dungeon magazine adventure, we were tasked with assassinating her. We split the party in 2 groups to case the area, myself and another character were in the throne room where she was holding court, I decided to take a shot, and unleashed a Split Ray Disintegrate spell. She passed her saving throw and took 10d6 damage. I hung my head, knowing we were about to die, then remember we were on the Astral Plane, which enhanced magic to where you could cast a quickened spell once per round, and I had a second S.R Disintegrate. This time she failed, and ended up dust. One shotted the Lich Queen.

[Brolof]

A friend of mine built a megadungeon as an easy thing to run for our little group. While traveling through it we went through a door... and got ambushed by a pair of Baykoks. Those things tore through our party like a knife through butter, almost taking multiple of us out while only two could really respond. They managed to focus fire one down, but the other was still rearing to go so the party retreated. I decided my gnome bloodrager, wasn't going to retreat. So she went into her rage and picked spider climb. She climbed up the wall of the room, jumped off the wall and grappled the thing mid-air to bring it to the ground. Then she finished it off. That, was the best I've ever felt killing a monster... My campaigns tend to not last long enough to really build up for a BBEG.

[DeathlessOne]

I try not to 'feel' any kind of joy when a character is taking the life of another creature. Some sort of grim satisfaction when justice is finally dealt, sure, but I mostly feel pity and sadness that things had to unfold this way.

[Bjørn Røyrvik]

In an alt.u. Dragonlance campaign the Dragon Armies had a task force set up to kill us. For a frustratingly long time (better part of a year real time, with weekly gaming sessions) we were subject to regular scry-and-nearly-die, having to stay on the run, avoiding any allies, having our plans foiled, etc. When we finally managed to take them out we the relief, vindication and vindictiveness in the room was palpable.

Finally killing a certain rival mage in the same campaign who for a long time stole/tricked us out of loot, foiled our plans and beat us when we tried to take him out was a close second.

As complementary stories, how about the one kill that by rights should have felt good but didn't?
In one game it was the bastard behind the brutal murder of a PC's daughter-in-law and grandchild. The full story of who and what and why would result in a serious wall of text which I won't bore you with, but the end result left a feeling of ashes in my mouth on behalf of the PCs. It was a bad end to a worse situation.

[Klorox]

Killing Rasputin in Reign of Winter was a big bonus to what was overall a dissatisfying campaign, I would have killed Baba Yaga too, but couldn't.

[Archimedes The Great]

In the Glassworks of Sandpoint all the way back at level 1 in ROTRL. My GM ruled that my Tiefling alchemist who had 5 fire resistance could spend a full action dunking his hand into the molten glass and throwing it at a goblin as a splash weapon.

It was just a perfect situation where I got to use multiple mechanical features in a very unique way and also got to specifically interact with the environment around me.

The imagery and uniqueness of that kill really made me feel like I wasn't even playing a game at that point and I applaud my GM for allowing it to this day.

[Asmodeus' Advocate]

Probably back in the ninth grade when I murdered my gym teacher and buried him behind the school . . .

Oh! You meant in D&D.

[David knott 242]

I was rather happy at killing an evil noblewoman who had tried (but fortunately failed) to burn to death all three children of her younger brother in order to move herself up in the order of succession.

I had a rather personal stake in that one, as I was playing the oldest of those three children.