For some people, they have a 180 degree change in personality when they drink. It's relatively rare but it happens. I have a cousin who is a calm and law-abiding computer nerd, but once he has a few drinks in him, he starts pulling fire alarms and breaking windows. He never was a big or regular drinker, but obviously alcohol and him don't mix well.
In my work, I deal with people who got in trouble for drunken behavior all the time. Most people will simply have an exaggeration of their pre-drunken mood when they drink. A happy person becomes a giggly person. A sad person starts crying. And a rowdy person becomes reckless.
Here, the guy may have been an idiot to start, or he may have been one of those 180 degree guys. Interestingly, people with certain genetics seem to be more prone to the 180 degree guy. My cousin was 1/8th Cherokee Native American, and he chalked it up to that, rightly or wrongly. Now, he doesn't drink even a single drop--and his life is better for it. Smart.