You can't say it was definitely not a factor in how he ended up where he did. Paranoia and disengagement could well have been exacerbated by his use of cannabis and had an overall impact on his mental health.
I think it is inarguable that cannabis does have long term side affects when it comes to mental health.
Though I smoked it all the time as a teen and all it did to me was make me eat lots of pizza and not want to get off the sofa, and laugh at unfunny movies.
Depends on the degree with which it is abused, and any pre-existing psychosis of the abuser, which, as you say, could have been exacerbated by cannabis abuse.
On the whole though, alcohol, a far more socially acceptable and legal drug, does far more harm to mental and physical health.
Even though I don't really partake, I would be in favour of cannabis legalisation, purely because there are other, far more dangerous drugs, that are legal.
To make cannabis illegal whilst allowing alcohol to stay legal is ridiculous.
But then I would probably legalise all but the most harmful drugs (Heroine, Crack and crystal meth), purely because there will always be a demand for them and at least if they were regulated, taxed and had appropriate age restrictions and health warnings, it would make them safer.
Anyone who wants to smoke cannabis will be able to, its pretty easy to get hold of, spending millions trying to police it is fighting a losing battle.
Same with prostitution. You will never get rid of it, its the worlds oldest profession and there will always be a huge demand for it.
At least if it was legalised and regulated, you would know that the women in the "bordellos" were a) of legal age b) Paid a fair wage c) doing it willingly and not being coerced or tafficked from other countries and d) had a clean bill of health.