Which is the most logical placement for sentence 6 (reproduced below)?
Things weren’t peaceful, though, as tension between the groups built and the protestors from Unite the Right became violent.

This answer is correct because the sentence in question works best as a transition between the first two paragraphs. While this change does not change the sequence of sentences in the passage, it does affect the meaning of the sentence. Sentence 6 makes more sense at the end of the first paragraph because it links the ideas of the first paragraph with the ideas of the second paragraph.