It's probably Cet laughing seeing as you've decided to walk down the "dark path" by killing the good NPCs.
Yep, that would make Cet laugh.
I wouldn't know from firsthand experience as I've never killed him so just a guess on my part. I think originally the designers did intend there to be consequences to players not maintaining the oath they took way back in the Crypt so most likely it is a left over from that which didn't get implemented in the release version.
Wyv