The context is that Trump is trying to provoke the kind of ideological skirmishes in the streets that he thinks will motivate his base to vote for him in November. Thereby fanning the flames of fascism.
So? First, that’s a result of white supremacy, anti-Mexican xenophobia, and neocolonial wealth disparities. Second, he didn’t say that some were rapists. He said that everyone who came from Mexico was bringing drugs, everyone from Mexico was bringing crime, everyone from Mexico was a rapist, and some of the people from Mexico doing so were, he assumed, good people.