The Scarlet Letter by Nathaniel Hawthorn

Here was the iron link of mutual crime, which neither he nor she could break. Like all other ties, it brought along with it its obligations. [Hester Pryne and Arthur Dimmesdale are bound together by their crime]

May God forgive us both. We are not, Hester, the worst sinners in the world. There is one worse than even the polluted priest! That old man's revenge has been blacker than my sin. He has violated, in cold blood, the sanctity of a human heart. Thou and I, Hester, never did so! [Talking about the malevolent Roger Chillingworth who is psychologically torturing the minister]

...the infectious poison of that sin had been thus rapidly diffused throughout his moral system.