Gun control is an issue imo. Yes Im sure there are other means by which crazy people could kill, for example an explosive, it would involve alot more work then just going to an gun store... not to mention concealment of large enough packages to do major damage would also be an issue, certainly not as easy as a small handgun. Same applies to a knife, no way are 30 people going to be killed by one guy with a knife or even a sword, they can just run away or shield themselves - luxuries which you dont have against a firearm.
And just look at Australia, Im sure there are people who are loners like that guy, if not worse. But with the inability to act on their emotion everyone is kept safe! (And yes you can have a handgun in Aus... but our gun laws are very strict, you have to have a purpose for having the weapon and go to things like shooting ranges etc on a frequent basis to demonstrate it... in the U.S no such framework is in place, you just get a gun for your own personal enjoyment)