Aside from the completely understandable grief at the tragic and untimely loss of a friend, and especially in this manner, at his own hand – and you have my deepest sympathies for that – you may be getting unwanted reminders of your own mortality. Speaking from my own experience and observations, young people simply don’t realize that they’re going to die. I never did. (I still joke that “I plan to live forever; so far, so good,” but I realize it’s a hollow joke – most of the time.)
Because of the ways in which we’re generally so insulated from death in Western cultures these days: people don’t often die from illness in their homes and in the presence of their families; suicides are kept quiet because of “shame”; infant mortality is greatly reduced (which is obviously a good thing! but still, it keeps us from experiencing death around us), and old folks die at nursing homes and hospices, it’s not very well recognized as “the other terminus of life”. What you’re experiencing is part of the maturing process: the recognition and awareness of Death.
Parents, especially new parents of their firstborn, are often incredibly fearful of every new sound their baby makes, worried about every news report about child abduction, baby-switching, strange and horrible ways to die, etc. They get over that. (Having the second baby helps, as does sleep deprivation and the simple need to get past it and develop a life again.) You’ll get over your current feelings, too, but I’m sure you’ll remember your friend as well, just not the end of his life so much in the front of your mind.
The only real cure, I think, is to keep on living. And to be mindful, rather than fearful. That’s like “being afraid, but grown up”.