More or less. Plus, Harry has a major impulsive/righteous streak that can lend itself towards the darker aspects. Hurting people who have it coming. It could be less about a specific aspect of magic, and more a general psychological leaning.
Besides, how would Cowl and Bob know Harry has a gift for necromancy before he did it? (I may be misremembering when said statements were made)
I don't remember either saying it, not saying you' re wrong, just don't remember it. Harry did feel all kinds of dark magic around Cowl though, said just being in Cowl's presence had a greasy dark magic feel about it. Evil Bob was a devoted Kemmler disciple so he may have been lying... Remember though, the Loa said Harry was "stained with dark magic." Harry admitted that he was, but most of it wasn't his but what others did to him, but said some was because he has made a couple of bad calls. The Loa declared him honest and agreed to help him by answering some of his questions. Don't think it would have done that if Harry had a real predisposition for evil.... As far as that goes a gift for necromancy doesn't make Harry like Cowl, just as being a gifted marksman doesn't make you a murderer. The ability doesn't have to translate into the act.
Yes, Harry is impulsive and righteous, but that doesn't have to translate into dark magic... In my opinion, for Harry, dark magic is something he'd use only as a last resort when all else has failed.. He is powerful enough that he doesn't have to resort to the dark arts.. Another element is both the influence of Eb, and the disposition he inherited from his father, Malcolm.. It is significant that it is often repeated that Harry inherited Malcolm's way and his good heart. May have been one of the reasons aside from love that Margaret chose him to be the father of her star born, his natural goodness tamps down the megalomania and other impulses that comes with the powers of a star born..