It's not really so much control and mastery, it's more like the very opposite, if you are drunk (and being a severe alcoholic he could drink far far more than most people could without passing out) it just comes as an impulse. I started self harming out of nowhere the first time I got drunk. I had like 2 shots and a kitchen knife suddenly looked like the most pretty thing on earth. Your skin feels like absolutely nothing, you have the sense you are painting, or watching a surreal music video, none of it feels real.
I don't know why only some people are like this. It's not all to do with being depressed. My theory is, some people - eg BPD, severe ADHD - who already have a weak emotional 'filter' can easily become overwhelmed by emotion to the point where other things - like self preservation are at least temporarily drowned out. They are already a little naturally 'drunk', have less internal emotional inhibition, so the alcohol has a kind of volatile effect. The intense self loathing which has to be repressed constantly is only a thread away from crashing down, at any point, so when it does, it manifests in a physically violent way.