This Sombra-esque specimen is Nero, the Umbrum emanation of Innocent Sun. Originally created through her dark magic for the purpose of infiltrating the restricted section of the Canterlot Library (long story…), Nero was initially nothing more than a doppelganger of Twilight Sparkle to achieve that goal. But when he (then "she", I suppose) outlived that purpose, she disappeared, assumedly ceasing to exist… until a year later, when he reappeared in Sun's life, trying to kill her with his newfound mastery over the same dark magic from which he was created. His excuse for his moral abhorrence is that Umbrum, as few as there are, are born with malice in their hearts, and exist to do not much else but harm. And while that may be true, he unfortunately fails to realize the fact that he can still grow beyond his supposed nature, if such a thing truly exists in the first place.