Not every schizophrenic is homicidal and not every heavy marijuana user becomes schizophrenic. I have just known several people whose family members have been told by doctors that their mental state was attributable to marijuana use. I didn’t believe them until I saw that recent study. I always thought the family members were either naive or trying to cover up more serious drug use or hereditary mental illness.
On the other hand the decrease in IQ of those who use marijuana heavily before maturity is fairly wide spread, though not everyone suffers the decrease in IQ to the same degree. An 8 point loss in IQ appeared to be the average.
Then there's not enough of a causal link between early marijuana use and violent behavior to consider in this case. Could be a contributing factor, but chances are just as good or better that it's not.
Some kids also come from broken homes where there was poverty and violence present. Most survive it, and go on to lead lives as decent people. Some don't.
I'm not saying that your suggestion is invalid as an entry point of unraveling this case. I'm only saying that it could be a possible influence. Most kids who smoke marijuana don't go on to become psycho killers.