Why Did Nathan Constantly Rag on Ezra?

It was starting to annoy me by the time Chinatown came on, but Nathan was always harrassing and belittling and accusing the worst of Ezra. He was especially harsh on Ezra with the China woman, accusing him of taking advantage of her, when he has never, not once before, shown to take advantage of a woman in the past. But really, was anyone else annoyed with Nathan's holier than out thou attitude with Ezra, either be it when Ezra was playing cards with a little girl, to trying to show Buck how to sword fight, to setting traps, trying to help prostitutes get a better life, and obvious, giving his bed to a homeless China girl, and even footing the bill for her train ride back home.