I don't think you are wrong to have such a belief, and we can all think of situations in which people might come to the reasonable conclusion that death was preferable to life. There are of course religious, and not only religious, principles on which suicide is morally ruled out, but social stigma is not nowadays normally much attached to suicide, it seems to me. For example, relatives who assist in the death of someone are rarely now convicted by juries of anything illegal, and in a sense they are assisting in suicide, the suicide of someone who is no longer able to carry it out by themselves. Suicide itself is no longer a crime, in most jurisdictions, and there exists a long tradition in many cultures of respecting the decision to end a life when one no longer believes it is worth preserving.
I would not be overly concerned at feelings of guilt, because we often feel guilt for things over which we have no control at all. It is not as though in a fit of sudden despair when you were not available to be with him he carried out this act. He thought about it over some time, calmly considered the various options and likely eventualities, no doubt including your feelings in the matter and the effect his action would have on you, and came to a certain conclusion. I think we have to respect the decisions of our children, especially when they veer away from where we would like them to go, and not feel guilt as a result of them.
On the other hand, in the case of someone on medication and with mental health problems one is always worried about how far autonomy is at issue. Did he really have the ability to take a calm and measured decision, or was his thinking unbalanced by a particular combination of drugs, or indeed their absence? In that case one might be worried about whether prompt intervention of some kind might have brought about a different conclusion. Then guilt would be appropriate. From the account you provide, though, this is not the situation, and I am sure you would understand the nature of what was taking place much better than anyone else. There is no reason why suicide need not be a brave and defiant act, and you should have no compunction at so describing it.