To Kill:
Dark Templar Warp Blades and Spawn Broodlings in Starcraft. Great sound effect, and it must be the ultimate fear for those poor souls on the ground. With the former, you see your buddies getting dismembered by a one-shot kill blade, and both the blade and it's user are INVISIBLE. For the latter, it's like the process of an alien facehugger in superfast motion. And you get two baby aliens instead of one.
CQC-Dagger Kills in Metal Gear Solid 3. My advice is interrogate, then decapitate.
Enemy deaths in Perfect Dark/Goldeneye, mostly for the voices not the animations, though those can be kind of funny in a twisted way as well. ('Oh my GOD he's dead!', 'Jesus christ, someone hit her!'). There's also a glitch I remember where you could use the Reaper's mower function to put a human into an eternal seizure if you hit him enough- the ultimate torture!
To Be Killed:
Castlevania Symphony of the Night stands out for me here. Alucard releases a suprising amount of blood, then we're given an ominous jingle and some rather demonic laughter. 'Let us go out this evening for pleasure. The night is still young.' Is that from Bram Stoker's Dracula? I can't remember...