Bombs are absolutely not hard to make.
And obviously he has no respect for the law. His gun was checked. Then he went into the bathroom, grabbed his gun, and starting going LEEEROOOYY JEEEANKIINNS on the citizens nearby. Who's to say that he couldn't have hidden a bomb? I know that it is hypothetical, but using a handgun is not exactly the best or easiest way to commit an indiscriminate killing rampage.
Also, no matter how hard you try, you will not be able to repeal the second amendment.