because he otherwise sounds normal
I'm not an expert on people who are capable of committing crimes like this, but I know that masking is a big part of why people are able to get away with seeming 'normal'.
Alcohol does not drive people to do this. To suggest that it's somehow capable of completely obliterating a mask that's been kept up 100% up until that point is also unrealistic, at least in my eyes. Now if there were other drugs, maybe something like PCP - there's a discussion to be had there.