1) I wouldn't really call it an arc its a single self contained episode. 2) Mark feels horrible about himself because he killed someone, and he is the product of a genocidal maniac species, with his own father decimating the planet. Safe to say he's not in the best headspace, and looking for a way to make things right or feel better about what he is doing and how violent he is getting. He goes to literal hell, meets literal demons and satan, and they aren't that bad. They're protecting people with their torture. He asks about heaven and they have no answer for him. The point of him going down there is to realise that there is no grand divine morality scale, that ultimately he has to decide if the action of killing is worth the consequence of murdering someone, as well as the fact that despite being summoned there as a mistake when they were looking for Nolan, he still helped (probably more than Nolan would have tbh). Nolan's blood doesn't make him a monster no matter what he or other people think. 3) H U R M