The causes of mental illness are only very imperfectly understood. There could be a connection with heavy drug use in some cases but it’s fair to say that it’s hard to separate cause and effect. Genetic/developmental factors are undoubtedly more important.
My original point was very badly expressed, but certainly in terms of the general obnoxiousness of The Beta Band (not just, or even mainly, Steve) I think drugs had a major part to play.