But it's bad writing. The Emperor having hubris is not relevant to the fact that making Luke mad *does not give him a motivation to join Palp, it merely-wait for it- makes Luke mad*.He's not as dumb as is being portrayed. It's more of an expectation of what will occur, since, frankly, it happened in the past.
Secondly, the Emperor is myopic. He wants to destroy any threat, from Luke to the Rebellion. So, if Vader kills Luke, the Emperor wins. If Luke kills Vader, then his anger will consume him, the Emperor will capture his friends and use his fear to corrupt him.
I lose patience with you. If this exact scenario was in the PT, it would be called out for the bad writing it is.