My Little Pony: Friendship is Magic - and then Discord was betrayed, and learned a valuable lesson, probably making him a full supporting character now instead of an anti-hero.

Tirek more manipulated Discord into turning against the others. He more or less played to his ego and base instincts and promised him freedom.

I mean, it was pretty clear before this that despite his friendship with Fluttershy, his relationship with the others wasn't exactly ideal for him. I got the sense that he didn't quite feel like he was truly friends with them prior to this.

To Discord, Tirek was a friend.