The truth about alcohol (or other drugs) and changes in personality: “When he drinks, he becomes a monster” vs. “When he becomes a monster, he drinks”

According to conventional wisdom and Stevenson’s Jekyll and Hyde, drugs or alcohol can turn people into monsters, because everyone has a dark side, and the drugs or alcohol bring it out. But everyone does not turn into a monster when drinking or using drugs, because conventional wisdom and Stevenson have it backward. The more likely scenario is that some people switch to a monstrous alternate personality who likes to drink or use drugs.

Postscript (added later the same day): The obnoxious personality may drink or use drugs for one or more of the following reasons. First, the personality may have originated in imitation of someone who drank or used drugs. Second, the personality may genuinely like the alcohol or drugs. Third, the alcohol or drugs may incapacitate the regular personality, helping the obnoxious personality take control.

That third reason may look like Stevenson was right after all. But no, in the Jekyll and Hyde model or Freudian model, Hyde resides in the "unconscious" or is the person's "id"; whereas, in multiple personality, the alternate personalities are always conscious, behind the scenes, often monitoring what is going on with the regular personality, and looking for their chance to take control.