Of course all those things are true, but that's under the assumption that Luke is thinking rationally rather than out of anger. Anger at all the things the Emperor is saying. Angry that his friends are in a trap, and that his fleet is going to be destroyed, and it was the Emperor who set it all up. Meanwhile, Vader's been standing there, practically silent, blending in with the background. Luke's definitely going for the Emperor, and the Emperor knows that Vader will block the shot, thereby finally getting Luke fighting.