Yeah but you also have to remember Luke is more of a pacifist far more merciful closer to the true path of what a Jedi should be which holds Him back and that Darth Vader is no where near what Anakin was in His prime. Also Starkiller didn't "own" the Emperor He held Him back at best at the cost of His own life. And further more George Lucas has stated that the Legends version of Luke is the most powerful Jedi ever and that's the only canon where Starkiller can exist now don't get me wrong Starkiller is very strong. But Luke is simply stronger in every possible way.



If Luke was out to kill in ROJ He would've done so in fact He could've easily overpower both Darth Vader & the Emperor in a one on one. And keep in mind Luke is suppose to be stronger than Anakin who had the highest potential of any Force Wielder thanks to being the Chosen One Sidious knew this all too well it's why He had to get Anakin on His side with a false promise and weaken the Chosen One through cybernetics.