Because it's explicit in the lore of Star Wars that fear and anger are the emotional lures of the Dark Side.
They're equivalent because they're both temptations of the Dark Side, they actually are the same as far as the known mechanics of the Force work.
And Luke should have learned his lesson about being ruled by fear at Cloud City where it leads him to fail at the one thing he set out to do (save his friends), and he should have learned it in the cave on Dagobah where he saw the lesson that striking out in fear will lead him to the dark, but he didn't. You can't claim he learned it from the events of the throne room because the films have never shown that he did, only things that are actually in the films count not things you imagine.
Luke just doesn't attack a powerful Sith in that throne room, he fights with anger on his side, it's one of Hamill's strongest performances in the whole original trilogy, smashing repeatedly against Vader with the strength of blind rage and his whole body sells it. That is the Dark Side in action, that is Luke fighting with its power, right there at the very end of his arc in the original trilogy he cannot keep it out, only pull back from it once it has already put its hand on him.
He is. Snoke even says so to Kylo Ren. Snoke is not as clever as he thinks he is and he has surrounded himself with ideologically correct but incompetent minions on purpose because they're easy for him to manipulate. (And he really should have read the evil overlord list, boasting about how easy to manipulate your trusted lieutenant is whilst he's right there in the room is awfully close to #77)He looks and sounds like a little boy playing at being a warlord.