Complex sentence: Critics took issue with Styron using the "myth of the black rapist", as portraying black men as prone to sexual violence against white women was a longstanding racist stereotype sometimes used as an excuse for lynching black men.
Split into simpler sentences: Critics took issue with Styron using the "myth of the black rapist", as portraying black men as prone to sexual violence against white women. Suspected sexual assault was a longstanding racist stereotype used as an excuse for lynching black men.