I also have to agree that "Unthinkable" is possibly the best Doom story in recent years. Too many Doom fans focus so much on Doom the "Honorable" villain, forgetting that Doom's code of honor is medieval at best, giving him a lot of wiggle room for doing whatever he believes necessary to achieve his goals.
As was pointed out, at no point is it suggested that Doom's bargain involved selling Valeria's soul; merely that he kill her. So it's about Doom damning himself for power. ...Which I also consider to be in-character for Doom, as he's always had a serious blindspot for the acquisition of Power.