Yeah. Men are still, to a huge extent, discouraged from sharing and expressing their emotions as much as women (since that behavior is seen as inherently feminine). People still make fun of guys for crying and other healthy ways of expressing distress. The "strong silent type" still seems to be many people's masculine ideal. This is exacerbated on hate-filled sites like 4chan and 8chan, where guys who express feelings nonviolently are regularly mocked as "soyboys" or whatever; putting that sort of rhetoric alongside racist propaganda is, as other people have commented, an effective way to promote racial resentment and, in turn, violent outbursts.

The problem of discouraging men from expressing their emotions nonviolently is definitely not confined to these boards, however. I see tons of characters in media and people in real life who model the idea that anger or violence, while bad, are understandable ways for a man to express feelings (the same sentiment for women is much rarer). It might be bad that a man did something violent, but it doesn't undermine his very identity by eroding his masculinity in the same way that weeping and sobbing and being sensitive might.

I can't count the number of times I saw guys punching walls in movies growing up, often in circumstances where their characters were really feeling sad or hopeless or hurt. Women in movies under the same circumstances would often cry or be consoled. Or to put it another way, in a movie scene with a man and woman under extreme stress in a room together, I've never seen the woman punching holes in her own walls while the man sits on the bed crying. There are tons of other examples of stress reactions that are usually modeled in media along gender lines in this same way.

I think this stuff is self-fulfilling. Guys are discouraged from acknowledging difficult feelings except through stoicism or anger, and in turn people continue to associate that behavior with guys. When men on 8chan start buying into racist ideology and feeling like they can only express their distress through anger and violence, it seems inevitable that things like shootings will follow. The copycat effect with shootings that already happened only encourages it (see how this shooter cited the NZ shooter as inspiration).

I think things are improving, but I think that until guys are trained to express a full spectrum of emotions openly and without shame, and people in general are acclimated to supporting and validating men's feelings, it's going to keep encouraging this sort of snapping behavior.