Why don't we just make murder illegal, that way the criminals wouldn't be allowed to do it?  

The problem isn't the implement used (guns in this case), the problem is that there are people out there with so much hate in their hearts that they choose to do unspeakable things to others.  You could eliminate every gun on the planet, but until the actual problem is addressed, nothing will change other than the weapon of choice (sharp stick, knife, car, airplane, explosives, biological agents etc. etc. etc. etc).