First, I really appreciate your thought-provoking post about this. I love the smart people on DU!
I think the design goal of the cover image was to evoke/provoke/shock an emotional response rather than tell the story of Dzhokhar Tsarnaev to any significant degree. I think the designers assumed that most people knew the "bomber" was a monster before reading the article or seeing the cover. And, the designers probably assumed that few people knew his backstory. So, they chose the sexiest, most cognitively dissonant image of Tsarnaev available. How can this babe of a young man be so evil?!?
Like it or not... This cover is stirring the reaction the publishers wanted and will sell a lot of magazines.
BTW, the cover image of Charles Manson you posted is interesting... I haven't read the article, but I think the cover design actually humanizes (rather than demonizes) Manson by highlighting his eyes... Notice the lack of yellow overprint over the whites of his eyes... Neither Manson nor Tsarnaev posed for a photo shoot... So, the designers could only choose from pre-existing photos.