It’s a symbol. People are attracted to the idea that someone could coolly shoot an evil guy and (for a couple days) get away with it.

If he had hurt innocents or fumbled the execution (pun intended) he wouldn’t be so popular.

Also consider how our institutions are failing us. People feel, often rightly so, that the systems aren’t working for them. The supreme Court is openly corrupt and makes wildly unpopular decisions. Health care is a shit show. The police somewhat routinely kill innocent people and their dogs. Plus a bunch of stuff that’s not true but people believe. It feels like there’s no path forward, and then some smooth guy just shoots one of the perpetrators dead? Amazing.