Indeed, a quick look at the Mafia Wiki suggests as much:
http://wiki.mafiascum.net/index.php?title=Serial_Killer
"A Serial Killer is a player whose goal is to be the last player alive. They are aligned with themselves (meaning they have no allies)"
Again not saying much, but if we assume that this bomber has the same win condition and such then they will likely be a member of their own faction.