Steven Universe - Resources are dwindling on the homeworld

Imagine the mayor coming to your homeless shelter to say he's shutting it down because "times are tough." Then you walk past his house and see him relaxing in his massive pool, lighting a Cuban cigar with a hundred-dollar bill.

Why do you think Peridot was so confused and angered by that encounter?

That's the point though.

Before this, Peridot had gone on and on about how wonderful Yellow Diamond is, how she loved how objectively superior and logical she was compared to anyone else. In this scene, this very moment, Peridot realizes that Yellow Diamond was fueled by her own petty grudge towards the planet and the war. That the person she looked up to the most wasn't that person at all.

It was what finally pushed her to want to stay on Earth and help protect it.