I've been thinking about it, and I at least think I know why Nightflame, the first living black foal in Anok, became Nightwing the Destroyer. Perhaps it was because the pegasi of Anok didn't accept him. Sure, they coddled him, and they at first had great hope in him. But really, they didn't accept him. If you've read Nightwing's Journey, you learn that when he was a weanling, all the other colts and fillies, besides his best friend Hollyblaze, avoided him just because he was different. And we learn that he recieved his power, he had trouble wielding the healing starfire and that Jungle Herd and the other pegasi pretty much abandoned him. In other words, they treated him like some special item that could fix all their problems, and when that item didn't work, they threw it away like some useless, broken piece of trash. So basically they treated Nightflame like an inanimate object that could be tossed aside when broken. It makes m ethink no wonder he became evil. Imagine if the people around you treated you like some special tool that could be tossed aside when proved useless. And like I said, he wasn't accepted. The colts and fillies didn't like him, and the other steeds only liked him up to the point where he failed to wield the healing powers they wanted from him. What's worse is that they taunted him, calling him names like " dud " even though he could fly. I just think it's all this poor treatment that turned him evil.
However, this does not mean I think any of the evil and heartless things he did in the both the past and the present throughout the series is acceptable. And besides, some of you may think that no one else can make someone evil or soulless, only they can do that to themselves.
But think about it, maybe poor treatment towards a certain person can make them mean and bitter. Maybe the reason Star didn't end up like Nightwing was because he had his friends and supporters. Nightwing only had Hollyblaze, and if you've read the bonus story at the end of Stormbound you learn that sometimes Hollyblaze wasn't always a good friend.
This is just a wild theory I have, so it's probably silly. But it's an idea on why Nightwing became so cruel and evil.