The skull icon denotes the last player that killed you in the match.
It even shows up on friendlies who TK you

Hey Lith, welcome to the worst clan ever. Dragon and Doot have proved to be a waste of space when it comes to TF2 they are always playing stupid Conan and sulk like big girls when they get owned in a game that requires skill and co-ordination, oh well, see you online.

Myth or man? Some say he only comes out at night, others say, two fish and a scoop of chips... but thats another story.