If your guy gets crazy-angry or -rude drunk, there's a scientific explanation for it. A Finnish study suggests people who become violent when drunk carry a genetic mutation called HTR2B Q20.
Those who have it were found to be temperamental, very similar to having a passive-dependent personality profile (a personality disorder showing lack of self-confidence and self-reliance, making the person depend on others to take responsibility for his life). But with the influence of alcohol, those gene carriers become impulsive and aggressive. Apparently they have a small number of the brain receptors that control impulsive behavior, according to study author Dr. Roope Tikkanen. So yes, that could explain why they hit on another girl, became rude for no reason to a stranger, or wanted to start a fight.
But we don't know yet just how many people have this mutation, if your guy has it too (or another mutation/condition that yields similar effects), or if he's just really a bad drunk. If you're terrified of his behavior when he's drunk, let him know (when he's sober, of course). You can't just let him be mean to people or be destructive (to himself, to you, and his friends). Ask him what his alcohol limit is, and try to get him to agree with you that you should help him stick to that limit. That should make him not lose his cool and be the guy you feel safe with.
Follow Stephanie on Twitter.