Why did the white people hate the colored people so much?

This is a very complex question, even within the context of Chapter 15 and the church bombing. My best advice would be to read the analysis for chapter 14 and 15 at the GradeSaver link below:

http://www.gradesaver.com/the-watsons-go-to-birmingham-1963/study-guide/summary-chapters-14-and-15