This research doesn't support the author's thesis:
Political tolerance is the willingness to extend civil liberties to people who hold views with which one disagrees. Some have claimed that private schooling and homeschooling are institutions that propagate political intolerance by fostering separatism and an unwillingness to consider alternative viewpoints. I empirically test this claim by measuring the political tolerance levels of undergraduate students attending an evangelical Christian university. Using ordinary least squares regression analysis, I find that for these students, greater exposure to private schooling instead of traditional public schooling is not associated with any more or less political tolerance, and greater exposure to homeschooling is associated with more political tolerance.
It's Playboy, so I'm not sure I should have expected a quality article, but this is pretty much the author rambling about his pet theory without much rigor.
I'd like to see the actual statistics on crime rate of homeschoolers vs public schools rather than anecdotal cherry picking of some high-profile cases. I would bet the stats would not be favorable to the author's thesis. What I do know is that almost all gun violence happens in inner city gangs, and inner city schools are notoriously bad environments of miseducation. It's also clear that statists despise homeschooling in principle because individuals are given a choice about the values to give in instruction.
What are some other traits that mass shooters seem to share? Broken families and antidepressant usage. With a topic like this, there are so many possible causes to blame depending on your political inclinations. Writing a balanced article is almost impossible, but this article certainly doesn't help the discourse or get to the truth.