It depends.
Do drugs cause violence? It depends on the person. It also depends on the drug, on one some drugs cause people to relax more, while others cause the exact opposite. People can also have different experiences while taking drugs, someone can be violent and others can be relaxed. The environment also contributes to the person's behavior.
Does mental illness cause violence? Again, it depends on not only the type of mental illness itself, but it depends on the severity. There are many different types of mental illness, ADHD, Anxiety, etc.. these are just some of the mild cases of mental illness and typically they do not cause violence. On the other hand, people who have schizophrenia could cause harm to other people or themselves. People who suffer from this disease often hallucinate people/objects and are influenced by the hallucinations.
Short Answer:
It really depends on the situation for both of these questions, this question is more of an opinion question and more open to suggestion. I hope this helped you form your own opinion!