The act of 'killing it' would be performed after a joke is being told when everyone has just got done laughing and 'the killer
tries to be funny and adds on to the joke in attempts of making it funnier. This of course is not funny, and in turn, 'the killer' was just a big buzzkill
, otherwise known as 'killing it'
Person 1: Dude, Blake thinks he's so funny but no one ever laughs at him...
Person 2: Seriously, he tries to make everyone laugh, but all he's doing is killing it.