Sean McGinnes didn't do that much wrong in Hell on Wheels, at least not from what we've seen with our own eyes on the show. He lied on behalf of his patron Durant in the hearing that almost removed Bohannan from the railroad. He enlisted his brother Mickey to help get rid of the Senator that Durant shot and killed in cold blood. Not all that much to have a tortured soul about, but when a lawman from Boston shows up, investigating what happened to the Senator, Sean starts falling apart. He's going to admit his crimes and sins, he tells Mickey.
Mickey's response that Sean's confession would implicate Mickey falls on deaf and vengeful ears. So when Sean goes to confess to Ruth, and puts his hands on her when she doesn't want to listen, Mickey pulls a Durant and shoots his brother dead. Ruth pretty much realizes that Sean was no threat to her, but Mickey assures her that he was saving her life.
The last piece of this morality play falls into place for Mickey, when the Boston lawman accepts Mickey's story, because Sean apparently killed two young women in Boston by strangling them with his bare hands. This also explains why Sean's conscience was so quick to act up tonight. But we the audience saw what happened between Sean and Ruth. The result? Mickey, like Durant before him, is getting away with murder, and only Ruth knows deep down, if she allows herself to think about it. In effect, Hell on Wheels is a Canterbury Tales set in the building of the railroad, and we've just seen the story of the murdering brothers.
Meanwhile, Bohannan mixes with Mormons and Chinese workers, Elam goes predictably ballistic when he learns what Eva did with Rose, and Louise takes Eva under her wing. Simmering, explosive ingredients for the final two episodes of the season, when the Swede, who is off somewhere offscreen, will surely show up and be noticed by Ezra and Bohannan.
See also Hell on Wheels 3.1-2: Bohannan in Command ... Hell on Wheels 3.3: Talking and Walking ... Hell on Wheels 3.4: Extreme Lacrosse ... Hell on Wheels 3.5: The Glove ... Hell on Wheels 3.6: The Man in Charge ... Hell on Wheels 3.7: Water, Water
And see also Hell on Wheels: Blood, Sweat, and Tears on the Track, and the Telegraph ... Hell on Wheels 1.6: Horse vs. Rail ... Hell on Wheels 1.8: Multiple Tracks ... Hell on Wheels 1.9: Historical Inevitable and Unknown ... Hell on Wheels Season One Finale: Greek Tragedy, Western Style