I think Nightwing became evil because of what happened to Hollyblaze.
In Nightwing's journey, it's said (well, written) that they were looking at baby crocodiles together (well, Hollyblaze) when the mother comes back. Hollyblaze and Nightflame flee, on hoof because they both know that Nightflame can't fly yet. However, Nightflame suddenly relizes thag he is no longer running beside Hollyblaze. Then he hears screams and shouts. He suddenly sees his best friend, fighting the evil, power-obsessed overstallion, Moonwing. "Nightflame!" Hollyblaze screams. "Help me!" Though Nightflame knew she didn't mean to help her escape; Hollyblaze meant for him to help her fight.
But instead, he whipped around and jumped into a bush to hide.
"Nightflame!" Hollyblaze whinnied, before Nightflame heard a sickening crack.
And then he realized that his friend was no longer with him.
Maybe it was out of pure grief did he become evil. Pure grief that he chose himself over his best friend, that he made the wrong choice? There were stories passed through his herd that he was the reason she was killed. Maybe he just couldn't take it, and that"/ also the reason why he fled Anok when he first recieved his power, because after hearing all about it from his herd, he couldn't believe that he caused her downfall. So he fled to deal with his grief, and when he came back, he looked at his herd with hatred and disgust, and probably thought that they were wrong. No, definitely.
So, just like every other herd at the time, he roared his silver fire on them.