You know, all the characters had faults. Egwene wasn't EVIL for being a bratty child who thought using magic was awesome, in the beginning.

But she never grew up. She got more power, and more responsibility, and more knowledge, but never more maturity. And that just twisted her into, well... evil.

I think the big thing is that she never really had anyone she respected who could give her criticism. Moiraine filled that role for a lot of the cast, but Moiraine was just another obstacle, to Egwene. Mat had his soldiers, and Tuon. Perrin had Faile, and Wolf Guy. Rand had his girlfriends, and Moiraine, and Lan, and a few others.

Egwene had... what, the Aiel? Siuan? But she never really seemed to take their lessons to heart, beyond how to get power. At best, her moral compass came down to that Tairan saying, "Take what you want, and then pay for it."