Theoretically, this one doesn't fit in the "kill" category, but at the moment I thought it was kinda cruel (and yet hilarious at the same time):
In GTA San Andreas, there's this mission where you have to take some gang members with you, sneak into a graveyard, and kill some opposing gang leader. So I brought some homeys ("Grove Street for life homey, you down?"), killed the dude, and then had to hurry the f*ck outta there. I grabbed a car parked in the graveyard and drove off, one of my gang members running behind me trying to get in.
Instead of stopping, I just yelled "Grove Street for life homey!" at the poor guy, after which he was brutally gunned down.