So I got into L4D real hard and looked up the character wikis and voice lines and something something, the Witch crying reminds Nick of his ex-wife and the wedding area "brings back bad memories", something something, "it's not easy to get blood out of a wedding dress" "last time I saw this many crying women was at my wedding" and uh, what the fuckity fuck happened at his wedding????
6
his wife exploded
my best guess is that, during his wedding, his wife said something he REALLY didn't agree with, and out of rage he just shot her with a gun he conveniently had in his pocket. i guess it's just another reason why he was (probably) a convict.