I don't know... I think some classes of drugs amplify someones shittyness. Call it Inherent Shittyness (IS). Time is a big factor, too. The longer they are on drugs, theore shitty they become.
Δ Shittyness = IS x drug amount x duration
Sometimes people get off illegal drugs and become decent again, but
I also think that everyone has a Decency score, where
Net Shittyness (NS) = IS + Decency
Some drugs remove the Decency filter such that NS = IS. If a person knows they have a high IS, and that, say, alcohol temporarily lets that show through, and they care about their perceived NS, then it behooves them to stay away from it.
They’re shitty before the drugs too.