He is beginning his supporting details by repeating the phrase “isn’t this like condemning….”
Parallel structure is the repetition of a specific grammatical structure to create a cohesive list. It can also include the repetition of beginning words or phrases to help create the parallel structure. Each of the rhetorical questions King asks that starts "Isn't this like condemning..." is a supporting detail to show that throughout history people have made peaceful advances for good that precipitated violence. These people would not now be seen as criminals or worthy of condemnation. He is arguing that his peaceful actions are not to be condemned just because others choose to be violent as a result.