You're absolutely right. Sometimes you can't regulate crazy. But sometimes you can. The truth is that you don't know that this (or any other) shooter would have resulted to a different method of killing. And in fact, from a criminal psychology perspective, it takes a different kind of person to use a gun than it does a knife or a bomb. Also I love how when we have 20 mass shootings in less than 12 months, the best argument you can come up with is that 80 years ago a guy used explosives to kill a bunch of people.
But in truth, your argument is somewhat of a separate argument than what the study is making. The study only briefly touches on mental illness, but is rather pointed in its rejection of many of the common misconceptions (and arguments) held by the gimme-my-guns-at-all-costs crowd.