I think the lesson the episode was going for was while it is important to control your anger, bottling it up isn't really a good idea either. The problem Starlight was having is in the past her way of dealing with anger was to blast the pony responsible with a bit of dark magic and teach them a lesson, however now that she has reformed she no longer wishes to bring harm to other ponies but has no real idea how to deal with such issues without relying on the use of her magic.