I kinda read "Porphyria's Lover" as a less Machiavellian version of "My Last Duchess." The guy is paranoid, fearing that this sweet girl is fooling around behind his back, and it drives him to kill her. The poem seems to be a condemnation of this man's tendency to desire domination and control over the woman in his life. If it did cause scandal, it would be scandalous among those who read this poem without looking for the meaning behind it. It would be crazy to assume that RB was advocating such an action, so we have to figure out what he was trying to say. Most people don't like to do that and are therefore scandalized by poems like this that try to address serious issues. Anyway, that's my take.