Of course he is responsible for his actions. We all are.
Recognizing that someone is an addict and having empathy for the tragedy of seeing them destroy their own life doesn't mean you are advocating they are not responsible for what they do in the grips of that addiction. In no way are you minimizing the impact to the innocent victim. I personally don't see any incongruency in feeling sad for the victim, his family, Barton and his family.
If people think addicts are just bad or weak people then they really don't understand the issue. It's a disease and when it gets a hold of people it's tragic all the way around. Tragic to watch them so filled with self hate that they destroy their own life and then in cases like this where that destruction spills out and impacts innocent others.