I've read some commentary on the internet that Elliot Rodger, the most recent mass murderer, killed people because he was taking SSRI anti-depressants. I strongly suspect these commenters are NRA activists trying to distract from the issue of the availability of firearms. However, it is worth discussing what role, if any, psychiatric medications could play in an act of violence.
SSRIs don't cause violent acts to be committed by people whose only disorder is depression. Unfortunately, many people who receive prescriptions for SSRIs have other disorders. People with bipolar disorders (manic-depressive illness) may present to a psychiatrist with depression, and upon taking an SSRI, become manic. Mania can include symptoms of grandiosity, aggression, hostility, impulsivity, and psychosis. Needless to say, these symptoms can trigger violent acts, even homicidal acts.
Many people who have depression also have personality disorders. Narcissistic Personality Disorder and Antisocial Personality Disorder (sociopathy) have as symptoms a lack of empathy for others. People with Antisocial Personality Disorder are known for frequently breaking the law. When these individuals receive SSRIs, their depression often clears up, and they have more energy. This energy can be manifested in impulsive, sexual, aggressive or criminal behavior. It isn't necessarily mania, but an elevated mood combined with a lack of regard for others.
I read parts of Rodger's "manifesto." His thinking seemed quite grandiose through much of it and at times crossed the border into delusional. I don't believe his revenge motive; rather, I think he decided he needed an excuse for his violence in order to justify it to himself. Rejection and bullying seemed like good justifications to him. I think that he killed people for the thrill of power and perhaps to gain notoriety. (Note that he killed men as well as women and none of the people who rejected or bullied him during childhood). He then killed himself to avoid arrest, condemnation, and jail. He knew he would never fulfill his grandiose desires, and decided that without that fulfillment, his life wasn't worth living. He wasn't going to get a blonde girlfriend who looked like a fashion model, and he wasn't interested in looking for a girlfriend who was someone more like him. This type of thinking is common among people with narcissistic problems and is the reason so many people with Narcissistic Personality Disorder present with depression. They can't fulfill their fantasies, and when reality hits, they are crushed. Rodger probably assessed his available options for achieving fame and power and came to the conclusion that mass murder was his one option.
What we need to do is keep guns out of the hands of people who are seriously emotionally disturbed regardless of whether they have been hospitalized or ruled mentally incompetent. Many severely disturbed people who are capable of extreme violence are also capable of day-to-day functioning and capable of lying to cover up their plans.