Remember that people's emotions are not trivial problems that you can solve if you just spend enough time working on them. The tech culture is particularly toxic, in my opinion, for people suffering from depression. Depression is not something you can sit down and hammer out a solution to over a weekend. It grabs hold of people and doesn't let go. It takes years of professional help (from people who actually understand human emotions and psychology) and the will to get better.
Please don't think about this too hard, because you can get caught up in all of the "If only I'd have called him" thoughts.
If you feel guilty, then talk to someone who understands how to deal with these kinds of events. Even a single visit to a psychologist can do a lot to break this feedback loop of guilt. Talking to someone who knows how to listen about real feelings of guilt or depression is the best thing you can do in your position.