Mass shootings are a vanishingly small percentage of homicides annually and merely point up that predicting violent behavior is something psychology is self-admittedly bad at and there's not much way to fix the situation in a way that respects the rights of the majority of the mentally ill. 

And the data don't actually refer to "lawful gun owners," nor does it show that the majority of the perpetrators actually bought the firearm they used through normal channels. It simply means that at the time of the offense they were, legally, "non-prohibited persons." 

That someone has not yet been convicted of a disabling felony, a misdemeanor Domestic Violence offense, or been adjudicated mentally ill does not mean they are a "law-abiding person." When you look at the prior criminal records of most offenders you will overwhelmingly find these are not "normal people" who just snapped, but people with histories of criminal conduct, often juvenile for the ones just entering legal adulthood.