I usually say nothing because, at the end of the day... You only killed someone in a game....
I'm the same way. I play games to have fun, and generally play "by the rules," meaning I don't grief, I use teamwork and I'm not out to "pwn noobs." - I treat others how I would like to be treated. When I kill a player, it's a done deal - no teabagging, no celebrating, no name calling, trash talking. A kill is a kill - just part of the game.