The lede of your link appears to disagree with you:

> Most individuals with psychiatric disorders are not violent. Although a subset of people with psychiatric disorders commit assaults and violent crimes, findings have been inconsistent about how much mental illness contributes to this behavior and how much substance abuse and other factors do.

No it doesn't; he didn't say most individuals with psychiatric disorders are violent. He said they are more violent than people with non-psychiatric disorders, which the article supports (especially if they also have a substance abuse problem).

everyone with substance abuse problems are more violent. correlation is not causation.

First, no, that is not true. Second, no one is trying to prove causation here, so it is irrelevant. Third, the existence of more violence is enough to justify avoidance, so causes and correlations can be left to policy makers and sociologists.