I won't call bull on those studies, but I myself was quite the marijuana user when I was a teenager, and I've never come close to killing anyone, nor have I ever felt a homicidal impulse in my life. I know lots of others who can claim the same.
Psychotropic drugs are an altogether different matter. There's a rapidly growing body of evidence that those kinds of drugs have a direct causal link to homicidal behavior. Here's just one such report:
Prescription Drugs Associated with Reports of Violence Towards Others
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.