What drives me crazy is the idea that someone who is about to commit mass murder (psychotic or otherwise) will stop and think to himself, "I want to kill all of these people, but dammit, if I do, I'll be violating a law against possessing this AR-15" (Or "I'll be violating this restraining order" or "I'll be violating the law against 30-round mags" etc.)
If all it took was a new law to stop mass murder or any form unwanted behavior, then the law prohibiting murder would already be sufficient. Laws against drugs would have resulted in a drug-free America. Laws against driving while drunk would have already eliminated drunk-driving deaths. But laws don't stop behaviors. They merely provide a pre-defined sentencing guideline.