Honestly, I think it's both. They are responsible for their actions. We all are. But those in active addiction are sick and not capable of taking responsible actions. Until they choose not to be, which is entirely on their timeline.
For me, this is less about what they are doing than about what my expectations are.
I used to get so angry at XABF for falling asleep on the couch with lit cigarettes all the time, but the real question is why I continued to expect he would behave any differently next time? Because I got mad? Yeah, my feelings about his behavior weren't even a part of the addiction picture. They were not enough to affect his behavior.
I had to remove him from my home and end our relationship in order to realize that whether he was responsible for his behavior or just caught in the whirlwind of an addiction didn't matter at all. The only thing that mattered to me was what I was willing to live with.
I had some people tell me that they thought I handled things badly because they felt sorry for him when I kicked him out. Then he blew up my social media sites, voice mail, and email with hundreds of messages calling me all sorts of names and accusing me of all sorts of illicit behavior that simply wasn't true, and he lost the sympathy of other people really fast. He was responsible for all of it -- but could he help himself? I dunno. I don't think so? In the end, it did not matter: unacceptable behavior was unacceptable.
Not sure if this helps in your situation. I guess I'm just saying I had to take the focus off the why's and wherefore's of the addict's behavior before I could focus on the only thing I could actually change: Me.