@flyawayxxballoon: You made some pretty extreme factual inaccuracies in your answer.
Depression is a very specific mental illness (I would be happy to explain this in detail), and it is not the “root cause” of other mental illnesses. Sometimes it occurs alongside other mental illnesses—psychologists call this “comorbidity”—but it absolutely does not cause people to commit murder. Saying depression leads to murder is pretty offensive to people who have suffered from depression and similar mental illnesses.
No mental illness is especially strongly related to murder because murder is such an extreme action. Sometimes people who commit homicides have suffered from illnesses like schizophrenia or antisocial personality disorder, and they are often the victims of extreme abuse and trauma as well. But none of these things leads to homicide.
(In the interest of full disclosure, I will mention that there is one kind of depression called “psychotic depression” in which people sometimes experience hallucinations and delusions that might lead them to commit homicide. This is extremely, extremely rare.)
Depression is not “caused just by being sad a lot”. When people suffer from depression, one of the symptoms is often a feeling of profound sadness, but sometimes people feel anxious, angry, irritable, hopeless, or completely numb. Everyone who suffers from depression experiences it a little bit differently, and it is crucial to understand that depression is not the same thing as sadness, even though they very often overlap.
Further, psychologists and psychiatrists who endorse mental health effects stemming from experiences in a past life will have a very difficult time justifying that approach. If you have a reliable source for this claim I would be very interested to see it.
I am sorry to hear about your friend and his family, and I hope that you will be inspired by their tragedy to learn a bit more about mental health. In the meantime, please tread lightly on the topic, and try not to make statements that you don’t know to be true. I would be happy to talk to you more about mental health if it’s something you find interesting.