Racism is bad because it makes us form groups and mistreat people that are different from us.

Doesn't mistreatment suppose there is some correct form of treatment, and wouldn't a racist believe they are using the correct treatment?

That is, I don't think this sentence is getting to the heart of why/if/when racism is bad. Your following sentence is closer but still not there- oftentimes, increasing differences between people is a winning move, not a stupid one.