They're on drugs or drunk before they kill more often than not. You can do all sorts of horrible shit when you're on cocaine without feeling anything. When the high wears off, you might feel a little guilty, so you just take more of the substance of choice to get rid of these feelings... and you can do all sorts of horrible shit again.
Now if you combine all this with someone who's a sociopath to begin with, it gets pretty nasty.
According to Murder Machine, DeMeo's liver was in the same condition as that of alcoholic street bums. He would have had to have been drinking at late stage alcoholic levels (i.e., 30+ units of alcohol a day - some get up to 45 or so) for that to have happened. So he would have been drunk or badly hungover constantly, even if he was good at hiding it.
thats interesting. I had heard that Demeo was a cocaine addict and Paul C. figured he was about to get caught and rat so they killed him. Never heard of the alcohol problems before.
I guess he was just a messed up individual in every way.
