Clinically depressed people should be given treatment and suicide should be prevented until the treatment (medicine and therapy, most likely) has time to work. When the worst is over and the patient is able to return home without killing themself, you can assume that they can fight suicidal urges enough to be able to make a responsible decision (if it's suicide, then at least people tried).
I don't support forcefully prolonging anyone's life if even medicine and therapy doesn't work, but if neither treatment has had time to work (a couple of months or so), the patient can't be considered sufficiently sane to make the decision to self-terminate.
I've told this elsewhere on the fora, but I've been diagnosed with depression and was treated, and it's been years since I got better, so I haven't been taking pills or seeing psychiatrists for several years now. I'm giving this background 'cause it may (or may not) put some sort of perspective behind what I said.
So in my case, if no one had prevented me from killing myself back when I was at the worst of my disease, it would have been horrible neglect. It was absolutely necessary, and definitely the right thing to do, to confine me and make sure I had no means of suicide available for the time it took for the medicine to take effect. After it did, my recovery was very fast; another proof that depression really is (mostly) chemical in nature.
But after the pills took effect and I was visibly over the worst of it, and I had arrived at a circumstance that made suicide the optimal choise - and in a way where I was no longer depressed - I believe I should have had the choise to kill myself, and I still do consider it my right if such circumstances arrive.
Quod gratis asseritur, gratis negatur.