Now that he's back to himself, Duke has the idea that he'll go and kill Croatoan but Croatoan can sense that Duke is trying to take control of his own mind, so Croatoan starts making Duke leak Troubles. Desperate times and the end of a series call for desperate measures and Duke begs someone to kill him. If he's dead Croatoan can no longer leech off his Troubles. After much reluctance, Nathan smothers Duke, one of his oldest friends. It turns out that Victor's vision from earlier in the episode was right – Duke did die in the hands of someone with a Guard tattoo – we just never in a million, trillion years thought that the tattoo would belong to Nathan.