Well for me personally it's because of empathy. The United States, as you rightly point out, was founded on slavery and genocide. I agree with you, and I would also add that the US is responsible for the deaths of millions globally in the past century (let's take Iraq and Vietnam, and I think you're already over a million). Generally a force for evil.
But a nation's citizens are not responsible for the actions of its government. They do not deserve to lose their homes in forest fires, have their children attend school hungry, go bankrupt on account of having cancer, be steeped in lifelong debt just to afford an education, lose their child in a school shooting, et cetera. What Elon Musk is doing will make those things much worse. That's why people are upset.