MSD

I do things that are supposedly addictive, but I can and do stop doing them when I want to. I'm also obsessive-compulsive, but I can curb most of those behaviors when they will be distracting, harmful, or undesirable. With the exception of some rare cases, I firmly believe that failure to fight addiction is due to weak willpower, not physical or chemical dependency on an action or substance.

When it gets to the point that the compulsion is to commit murder, I think it's possible that the imbalance and mental defect is so severe that either the understanding of why that behavior should be avoided or the ability to avoid that behavior is absent.