A common term in both internet and RL conversations. Someone who's sole ambition in life is to stop others from getting lulz or sometimes even epic lulz; often an internet user in a postion of power who tries to stop lulz-worthy acts of anarchy.
Person 1: WTF? Some n00b mod on wikipedia reverted my lulztastic edits to this page.
Person 2: What a Lulz Killer
Person 1: ZOMG this admin disabled my uber CS hacks; now I'll have to get headshots using only skill.
Person 2: o_O What an EPIC Lulz Killer