There are plenty of studies where antidepressants are linked with increased aggression and violent behavior.
Our brain is so complicated that what we call mental illness (e.g. depression, schizophrenia etc) usually is umbrella of different illnesses with different reasons with somewhat similar symptoms that are diagnosed under one label. That's why for one person one antidepresant is helpful, for another one different one. For some no drugs can help. He might also be depressed because he already has non-diagnosed cancer (one of cancer symptoms) or is poisoned with heavy metals. Or maybe something completely else. And it makes no surprise that if you throw antidepressants at someone who will react violently to it because of his current brain chemistry bad things will happen. Guns were available for long time in USA, would be interesting to see how antidepressant usage growth correlates with number of mass shootings over last 100 years...
Here is also interesting excerpt from the book "Nutrient Power: Heal Your Biochemistry and Heal Your Brain":
Most of the school shooters exhibited a unique and unusual history, compared to the thousands of behavior-disordered persons we have studied. A major difference is the absence of violent behaviors until the teen years, and many cases of excellent academics. Typically, the school shooters developed anxiety and depression after puberty and were treated with SSRI antidepressants. These drugs have helped millions of persons, but psychiatrists have known for years that a rare side effect involves development of suicidal ideation and in some cases homicidal tendencies. There is considerable published literature that indicates young males are especially at risk for this side effect. More than 90% of the school shooters we studied were treated with SSRI medications.
Mainstream psychiatry’s “treatment of choice” for depression is use of SSRI antidepressants aimed at increasing serotonin activity in the brain, perhaps coupled with counseling. However, as described in Chapter 6, depression is not a single condition but an umbrella term that encompasses several quite different disorders. Figure 6-3 shows the five major biochemical types of depression, including a low-folate phenotype that is associated with intolerance to SSRI medications. It seems likely that most school shooters had the low-folate form of depression and experienced an adverse reaction to antidepressant treatment.
These persons respond better to benzodiazepine medications, and also benefit from nutrient therapy to elevate folate levels. Another danger of antidepressant drugs is sudden non-compliance. There are several cases of school shootings in which the crime occurred soon after the offender stopped SSRI medication.
Recommendation: Doctors should perform blood tests prior to prescribing SSRI antidepressants for young males. Inexpensive blood testing for histamine, serum folate, and/or SAMe/SAH ratio can efficiently identify persons at risk for suicidal or homicidal ideation following use of SSRI antidepressants.
The chemical balance of the brain is extremely complex and there is no way to know of the side effects.
A lot of shootings in America can be linked to previous antidepressants usage for example.