Vader was after something else entirely. He could also have skipped the whole Empire and gone out for pizza. But that would not have achieved his goals. You are looking at that conflict in terms of Good Guys vs Bad Guys. None of the bad guys are looking at it that way. The Emperor doesn't care if one of his minions rules a united, secure and peaceful Empire after his death if he has to die to make it happen - the Emperor wants what he wants. The same goes for Vader: for him, the best choice is to be the emperor himself, with Luke serving him as he himself served the emperor. He wants that more than he wants a victory for the Empire. Luke can laugh off the Emperor's death & destruction of the Death Star if he dies too, because then the cause to which he dedicated himself is triumphant, and his friends win and he can be happy for them. The Emperor and Vader are selfish and evil, and don't care about their "side," only themselves. As we plainly see in the climactic battle, Luke is more important to Vader than the Emperor, so his actions are entirely consistant.