I think deep down you know the answer to your own question. But for the record, no, it is not your fault. Sometimes it's easier to blame ourselves than to think that someone has made a really bad choice and does not want to address it. For me, self blame was a subtle way of trying to regain control - if I was to blame I could fix it, if I was not to blame, only sadness remained.
But maybe there are other questions that might be more useful to you?
What might you do with your life if you weren't involved in all this drama? Why do you feel responsible for his actions? Relapsing is a choice and as they say, relapse is part of recovery. If something in his life was beyond his coping skills, his opportunity is to learn new coping skills, not blame you. And if he chooses not to mature in this way, that is his choice.
Pray, hope, don't worry and set boundaries. Good luck.
Sounds like the new guy deserves a chance. (my humble opinion)