It’s the result of the “bombastic” mix of false dichotomy, assumptions, and social media dynamics.

False dichotomy prevents you from noticing nuances, complexities, third sides, or gradations. Under a false dichotomy, there’s no such thing as “Alice and Bob are bad, but Alice is worse than Bob”; no, either they’re equally bad (thus both deserve to die), or one of them is good.

In the meantime, assumptions prevent you from handling uncertainties, as the person “fills the blanks” of the missing info with whatever crap supports their conclusion. For example you don’t know if Bob kills puppies or not, but you do know that he jaywalks, right? So you assume that he kills puppies too, thus deserving death.

I’m from the firm belief that people who consistent and egregiously engage in discourse showing both things are muppets causing harm to society, and deserve to be treated as such. (Note: “consistent and egregiously” are key words here. A brainfart or two is fine, as long as there’s at least the attempt of handling additional bits of info and/or complexity.)

Then there are the social media dynamics. I feel like a lot of users here already addressed them really well, but to keep it short: social media gives undue exposure to idiots doing the above due to anonymity, detachment from the situation, self-reinforcing loops (“circlejerks”), so goes on.