I am sorry you are dealing with this loss. It is so hard when people don't live up to what we know is possible for them. Sometimes the people we love behave like fucking idiots. N falls into the latter category obviously. I caution you about putting too much weight on your actions and if you coulda/woulda/shoulda done something differently. Nothing you did or could have done would have made a lick of difference.
See if there is anything you can learn from this relationship. Maybe that is only 'well, don't get involved with him again!'. That is more than a sufficient life lesson. Or sometimes people are stupid and hurt us unnecessarily.
Good luck. The pain will ease with time.