Leftists are shallow and stupid, so their idea of a hardcore right-winger, someone hard-hearted enough to kill on a whim, misses wide of the mark. None of us care about how "American" someone is. None of us like Donald Trump. Like American History X, this movie is by and for fools.
I imagine him being an Army vet who didn't want to sit on base and wait for orders, and neither did his brothers in arms. So they lead a "mutiny" against their diversity-hire command, cleared out the armory, loaded up a dozen trucks and headed out to restore order across the land.
The very last image of the actual movie, "Civil War", is three black soldiers, one of which is female, sticking their AR-15's in Not-Trump's face and making a bloody mess. They filmed the movie in the wake of 2020 but shelved it to release it before the 2024 election instead.
Anyway, the first Rampage was uncharacteristically good and the second was more than decent.
https://m.imdb.com/title/tt1337057/