Destroyer doesn't have to be a terrible monster. It sounds more like something out of The Book of Revelation, the end of the world, Judgement, Stars and Stones, Empty Night.. There are a bunch of angels running around doing terrible things under orders.. As a star born, Harry could be one of them.
True. But from context does it really sound like not a terrible monster. Morgan says "The thought of allowing
a Destroyer
to be birthed among us when I could have stopped it is
too heavy to bear."
I have bolded the intention points. There is obviously the possibility of more than one Destroyer, that could be born amongst "us" (I think he means mortals in general) and that allowing that to happen would be a disaster. It clearly reads that Morgan is very concerned by that possibility. And every other character that has hinted at it (Aurora, Mab, He Who Walks Before, He Who Walks Behind, Mother Winter, Martha Liberty, Nicodemus, Drakul, Cowl, Vadderung, the Gatekeeper, even Ebenezar in a roundabout way) pretty much couched it in terms of being a danger or a weapon.
I don't believe Harry can use it for good entirely, he never gets that lucky. If he can help people with it I suspect it's mostly by directing himself at the bad guys. It mostly sounds like he is a walking nuke. And Morgan hints that there can be more than one...