To "kill" something you need to do it very well. After accomplishing something with great efficiency you are recommended to pump your fist to show people around you that you "killed it" also to signal people from a far distance that you killed it.
tr. v. (figurative) also kill it, killin’ it
used when describing in brief an excellent or beyond satisfactoryperformance by any individual or team, used commonly to describe sports contests, musical performances, video game tourneys, etc
When someone takes something too far and just ruins the moment completely. We've all done it before, and there's always some jackass who yells out "KILLED IT" as soon as it happens.
Pete:I thought all the dinosaurs were extinct, but I'm looking right at a Fagodactyl.
Ryan:Yeah, well some homosaurus is staring me down right now.
Max:VAGINASAURUSREX Ryan:Killed it.