The objective truth is that Steve was an alcoholic and drug user who had serious mental health issues. After decades of addiction, he died of a heart attack.
Your subjective experience is that Steve, although deeply flawed, was worthy of love. Because you spoke to him soon before his death, and that he died alone, you feel that somehow you could have said or done something differently that would have changed the outcome. Because of his family's "norms" you had no chance to truly grieve and get some degree of closure.
I got sober twelve years ago today, and I am now 55. My life was probably not a whole lot different than Steve's when he was in his early forties. I seriously doubt that I would have lived to see 61 if I hadn't gotten sober when I did. Nothing anyone said to me changed my actions when I was in active addiction - it took me to a place not unlike Steves life: I lived alone, was unpredictable and selfish, and although I desperately wanted to have a life filled with love I was incapable of long term relationships. Alcoholism was my everything until I was willing to remove it from my life entirely. Addiction by its nature defies statistics, but in my experience very few people get and stay sober for the rest of their days. Steve's story is tragic, but sadly not all that uncommon. Nothing you could have done would have changed his trajectory, addiction or recovery was his alone to choose.
I think it's pretty universal the way people feel when they first hold an infant: awe. We are all God's children, and I think we sense that when we hold a child. That is what we grieve when we lose someone dear to us - it doesn't matter so much what they did, they still carried a spark of the divine. When that spark fades, the world loses a little bit of its light. Remember Steve's spark, his soul, and wish him well on his journey. Rather than dwell on what was and might have been, look forward and use what you have learned to live your life in a way that honors your spark. I think that's the best way to honor those we lost.