You make a good point. If there were no guns at all there'd be much less killing. The point is that the PA never succeeded in policing its territory. Nobody was ever strong enough or determined enough or interested enough in peace. The only reason that the IDF is there is because of the threat to Israelis. We're well able to police our settlers but if they're threatened by unpoliced weapons in the hands of Palestinians we can't ask them to give up their weapons.
from the article: Israel, PA resume West Bank civil co-ordination severed in Jan.