What makes prison inmates so violent?

Recently I watched an episode of Locked Up. There was a segment on the notorious 1980 New Mexico State prison riot. Due to overcrowding and many other issues, guards were taken hostage and some were brutally raped. What's worst was that inmates housed in cell block 4 (mostly informants and weak/vulnerable) were tortured, mutilated and even decapitated (blowtorch was used to cut someone's head off). I was in shock and had the same question some of the rescuers at the scene had: How could human beings possibly commit such things? Are these inmates naturally violent or did they become violent while in prison? One rescuer served in vietnam and even he said he hadn't seen something like that.