When someone that close to you dies from something preventable, you feel the need to blame whoever you possibly can. It is not your fault. He IGNORED the signs on his own, you didn't make him do that. HE did it. He probably feels the guilt, as well as the blaming feelings, but is just overwhelmed with emotions of anger and sadness. It's just feelings in him that he is trying to vent out in whatever way.
I think it is amazing of you to say that you will let him express himself and try to find empathy because I think that is kind of what he needs. Just to vent and get it all out. Please don't let it hurt you or make you feel worse- don't take it personally, he is overwhelmed and you know you are not to blame for her adult choices and his choosing to ignore the problem.