I guess at this point it is a matter of beliefs. I dont believe in causing another pain sbecause the caused me pain. In the end I know myself and that attitude won't bring me peace.
As far as likening an alcoholic to someone else with a terminal illness. Well I dont believe in extending the suffering but I dont believe I have the right to make that decision. Only the person suffering has that right. I have spent time twice with loved ones during their final days. There isnt always pain and suffering. There can be great peace and grace.
As for suicide. Your are assuming your father would be unsuccessful. I have family members who were successful the first time. They had spouses, children, and grandchildren. And no one suspected a thing.
In the end you will be the one who has to live with your consequences. I hope for your sake you chose to let go of your anger and walk away.