Like I said, you did not stand in his shoes. You do not know his pain, indeed his daily agony that had likely become unbearable. Would you want to watch a loved one live in agonizing pain day after day? Would you enjoy that? Or would you want to release them from their abject misery?
I had two family members commit suicide. In both cases, they did so to relieve themselves of terminal pain. One of daily mental anguish that lasted years and years, and created chaos, trauma, and misery in our home. The other because of a terminal illness. In both cases, I support their decision, 100%. They better for it, and in the case of my father, his family was better for it. It was the kindest most generous thing anyone has ever done for me.
If you cannot imagine mental pain, perhaps you can imagine perpetual physical pain due to illness of such intensity that after years and years of enduring and fighting it, you ask -- or beg -- the doctor to let you pass on. Mental illness can approach that same level. It can be, literally, torture. I'd like to see how long you would hold up under that kind of duress. If you are lucky you might have a support system, a loving family. Some people don't have that, and "the system" is not an appropriate surrogate.
I support assisted suicide. Let them die as they wish after appropriate counseling and support, and in a comfortable and peaceful way rather than making a mess.