TD... White guy shoots up a prayer meeting? White supremacist. Two white guys shoot up a high school? Nazi sympathizers... why leave the most egregious spree killers in the past couple of decades out of your analysis? That's not even getting into the attacks on, murders of and kidnappings of abortion providers in the past two decades - almost all perpetrated by white people.
I don't place National
Socialist sympathizers on the "right wing" side of the political spectrum. I live in the United States, not Europe. Generally speaking, I place most racial supremacy groups outside of the (US) political spectrum in general as they exist on a different axis than the traditional left-right political spread allows for, a position that generally puts them more in line with the left wing than the right wing(as once you remove the racial supremacy content, you either end up with leftist agendas, or cults, although I'll admit there are a few at first glance, "Constitutionalist" groups in that mix that would tend to skew things to the right.
They get lumped into the right-wing because where they do overlap with the traditional left-right spectrum, they're barely matching up with the American Right(in regards to "individual rights" which they of course reinterpret the "individual" part of that to be "white" and often more specifically into "white straight male"). But that's mostly because the things they oppose most strongly at present happen to be things the American Right-wing likewise objects to, just for different reasons.
But even if I accept your premise of them being right-wing, spree killing aren't
riots. The anti-abortion killings, aren't
riots. As a "riot" requires a "large group" to be involved in "violent behavior." Almost every counter-example you provided involved only 1 shooter. A small number involved 2 people, and an even smaller number involved 3 or more, which would meet Webster's technical definition for "riot."
Although I'd personally set the bar higher than that and require some degree of
indiscriminate property destruction to be thrown into the mix. Which makes a racist group shooting up a minority religious observance fail that test, and ditto for an anti-abortion activist bombing an abortion clinic, or their bombing of/shooting at abortion clinic employees. That isn't an act of indiscriminate property destruction or violence.
The American "right-wing" does get violent, but that violence when it happens tends to be highly targeted, and has generally been targeted specifically at the (correct) target of their ire, whatever it may be for that specific person. The acts are usually premeditated, and only happen after some degree of planning.
The "left-wing" on the other hand, just simply goes out and riots, and usually ends up doing great harm to their own immediate vicinity and people that either have nothing to do with what they're actually rioting over or only have a tenuous relationship to it at best(such as "not being black" or "being white" and being where the riots were in the case of some of the racial riots in US history). They occur at times of high emotion, are spur of the moment, and aside from an activist agitator possibly being involved(which I strongly suspect in the riots over the past several years), aren't premeditated(which is why their location selection usually sucks balls).
Which puts it back in the court of "If
rioting occurs, it will be in the event of a left-wing(Clinton) loss."
Now if mass shootings, bombings, or other such things happen, those could be
potential right-wing responses(as they're not the exclusive domain of the right-wing; Islamic Terrorists and leftists employ both tactics as well) to a Trump loss, but it is likely that such a response wouldn't happen on Election Night, or even the week following it. That response would come along later.
