I was thinking that too until I watched ROTJ again a few nights ago. Palpatine continually says, "It is unavoidable." He doesn't have to do anything. He's already done all the plotting. He has brought Luke where he needs to be with the people he needs to be with and to witness what he needs to witness (the destruction of the Rebel fleet). All Palpatine has to do is watch (with glee) the resulting screw-ups. And he does. Luke does give into his anger and almost does kill Vader. It's almost like a Roman emperor pitting his two best gladiators together. It doesn't really matter to him who wins. If Vader wins, Luke is dead, and their biggest threat is gone. If Luke wins, the Emperor gets a shiny new apprentice.