I'm sorry you are going through this. You are not alone.
It is normal to question yourself, leaving a relationship, but to leave one with an alcoholic leaves you with even more questions. It's hard to get your head around how they could let the people they love slip through their fingers and choose that way of life. You can't apply rational thinking to an alcoholic though, addiction is at the centre of their world and everything else is on the perimeter. Their brain is also altered by the alcohol.
When you start to question things, remember that no one wants a relationship with someone they love to end. It must have been pretty bad for you to make that decision, it's not a light one, and particularly when you still love him. Things would have had to of been really terrible.
It is fresh right now and you are grieving but what you are feeling is normal, not an indicator that you made the wrong decision.