As I said before, Q never killed anyone.
Didn't he freeze some crewmember to death?
No, the worst he ever did was allow the Borg to discover the existence of the Federation prematurely by flinging the Enterprise right in front of a Borg cube in deep space. Guinan even stated that if the Federation and the Borg had met when they were supposed to, then they would have been ready for that first contact and could possibly have negotiated with them (and yes, I know that subsequent Borg appearances puts this in doubt; but that's what she said). So Q inadvertently (or advertently, depending on your point of view) caused untold death and destruction just because he felt like showing Picard the Borg in person instead of just mentioning them to him.The worst he ever done was put people in harm's way.