I think the article linked by @antoncohen sums up the situation nicely:

https://medium.com/s/story/fearing-for-our-lives-82ad7eb7d75...

I don't think the reasons you have given should give the U.S police license to default act aggressively towards others - due to being in fear of their lives.

From my perspective, the U.S. police are not in fear of their lives through fear of immigrants or terrorists. They are trained to be in fear of someone having a gun pointed at them.