Henry, Sorry to hear about the loss of your co-worker and friend.
Often, the people who actually are able to do this give little indication of their intent to do so. Depression, dealing with life in this complex world, and feeling hopeless about whatever is was that was troubling his mind, caused him to make this choice. Some view it as a selfish act because of the pain it causes others, but I can assure you in his mind, he saw no other solution to his inner pain. Suicide is a very complicated trail to walk....most turn away from that path, but some do not. Although it will take some time for those left behind to understand his choice, take some comfort in knowing that he has found peace.