IMO. The killer was very sneaky. It's all about getting away with something that drives their ego. The killer probably gets away with a lot of other tiny things like: blowing thru stop signs, unpaid taxes, cheating on exams, traffic citations etc. Nobody calls him out and people always cover for him. Recreational drug user, gambling,likes alcohol. He likes that adrenaline rush. Possibly a kleptomaniac, robber etc. It's all about what he can get away with which likely includes conning other people, Jekyll and Hyde persona, " duping delight". It's more of a character issue than an actual personality disorder. He loves pulling one over on other people because he's not attached to anyone. He is a smooth talker, disaffected about society and could care less especially about other people. He probably is emotionally immature and likes to snicker behind people's back. Thinks he is a big man- he's the boss- " I showed those college kids" An insidious viper of sorts IMO