What's more likely?
a) He walks around to so-and-sos house with his legally purchased, unlicensed, unregistered semi automatic assault rifle and 1000+ rounds, and kills the whole family, and anyone else unlucky enough to be nearby.
or
b) He spends days (weeks?) making his own gun, gunpowder and projectile by hand, and is still motivated all those days later to walk around and kill everyone, with a small caliber, (probably) non-semi automatic, (probably) small/no magazine.
Gun control won't stop people killing each other with guns. It will make it many times harder for the average Joe, therefore it will reduce the frequency, and the overall homicide rate.