Tarot reveals he was mixed up with a high ranking evil friend had wine taste on a beer budget, he may have pulled a con job on this man, or used as a scape was facing some kind of litigation, and violent encounters. Someone encouraged him, handed him the gun and said this is the only way to pay off this debt or an older woman will be responsible and seemed he was stuck and no matter how much he tried to do good or better it seemed to turn out worse.
Hi Jina. Firstly, I am so sorry for your loss and that your friend went through this. The cards i pulled tell me that he was going through an internal struggle and couldn't see a way out. He was in a situation he didn't know how to get out of, and felt weighed down by past problems an responsibilities and felt this was the best way to let go. To answer your question, I believe he did this to relieve himself of his struggles.
I'm sorry for your loss Jina. I used Lenormand, not Tarot, cards for this question because they give just blunt straight forward answers to questions which is what I think is needed here. I pulled 2 cards and got 36.Cross & 24.Heart. This indicates an emotional pain and burden, heart ache that became too much to bare. My intuition tells me someone broke their heart, or was causing them a lot of emotional pain, and the cards I drew backed that up.