From A New Hope:
LUKE: I can't get involved! I've got work to do! It's not that I like the Empire. I hate it! But there's nothing I can do about it right now. It's such a long way from here.
At this point, Luke lived on Tatooine, a planet far away from most of the Empire, that had only very recently began being militarized.
Were there any specific events that made him out-right hate the Empire? If not, how did he find out enough about them to form this strong opinion?