“Manipulating ponies with ease in many episodes”
You mean doing it once.
Has she used her magic to brute force solutions? Yes. But after the one mind-control incident, which she was chewed out on, she was not manipulative in any further episode.
As for the topic on hand, Discord is not the brightest bulb in the drawer. He’s a schemer, he likes to get what he wants. Even when he has something of a moral center after season 4, he’s not likely to consider things from anyone else’s point of view.
As for comparing the two, they both have something in common: They are both rash and driven by emotion. That being said, their mindsets are different. Starlight seeks to solve problems, with a tendency to jump to the first solution she thinks of and trying that full-force. Discord mainly wants to do things that benefit himself, and even when he tries to help others often can come from a self-centered viewpoint with Fluttershy being one of the few he is truly considerate of.
Neither of these mindsets are inherently evil, but both can result in making huge mistakes from not thinking things through.