According to past conservative arguments, we can judge all conservatives by what this person did.
I've tried explaining to conservatives why they should not use anecdotes to judge entire populations, and every time I do, I am told that I am stupid and attacking their free speech rights. Since conservatives are never wrong, the logic of their arguments is therefore impeccable, and we should judge all conservatives by what bozos like this (he's far from the only one) do.
So now conservatives have to explain why they are a terrorist organization and explain why they aren't doing something to stop the terrorists in their own ranks.
Firstly, merely wearing a MAGA hat isn't proof of you being a conservative. He could easily have been a Trump supporter but who knows.
Secondly, this case looks like a crazy person rather than anyone motivated by political ideology. I mean, political ideology could always be a factor in the mix, but he may just be bonkers and politics has nothing to do with it. Not every crazy person is a "terrorist".
The threat of right-wing violence may perhaps be higher in the US compared to left-wing violence; but from what I remember of worldwide statistics, it's actually the left-wing that has been more dangerous recently.
"According to the report, the second largest category of terrorism perpetrators in 2011 after Sunni extremists was “secular, political, and anarchist groups,” which were primarily identified as Marxist, communist sympathizers."
https://www.cnsnews.com/news/article/sunni-muslim-extremists-committed-70-terrorist-murders-2011
And of course, yes, we *shouldn't* judge whole populations just by anecdotal evidence. But when it comes to a religion like Islam, we can judge it on things like, (1) the Quran and hadith (2) sources on the behaviour of the early Muslims (3) sharia law sources (4) commentaries on the quran by notable muslims (5) the behaviour of Muslims historically, taking into account to what degree they were commanded or inspired by their religion in their actions (6) statistical evidence for what kind of views are held by Muslim populations today (7) statistical evidence for say, whether Muslim populations living in the West are more of a terror threat than other religious groups, where any difference could be plausibly explained by religion.
Of course none of this amounts to "judging all by the actions of one". Judging a religion is not the same thing as judging all followers of a religion either. Of course you can judge the percentages that may be rather fundamentalist, if you have e.g. polling evidence for that.
You may be able to perhaps judge political movements in a similar way, but it wouldn't be fully the same, as e.g. political movements probably don't have a core document over hundreds of years that is supposed to be the "word of god" that you can't change without messing that up.