Bingowings said:

Vader hopes Luke will be take is place as second in command with himself as the new Emperor.

I've always understood that as Vader's overt motivation in Empire, but I actually like the idea of that as a motivation in Jedi too.

Vader brings Luke to the throne room so that together they can defeat the Emperor, but he can't tell Luke this directly because Luke is not interested in an imperial power grab... yet. But Luke fails to act against the Emperor on his own (fails to turn to the dark side), and Vader sees his chances of a successful coup dropping by the second. So he hastily grabs the Emperor and stages the coup single-handedly... critically injuring himself in the process.

So it wasn't an act of self-sacrifice, it was a simple coup gone wrong. Vader's "you were right about me" at the end was just as disingenuous as the deathbed confession of any bastard who wants to whitewash his record before it's too late. And Luke buys the whole act, the credulous schmuck. Blown up planets? Torture? Forget all that, Luke had a Special Moment with his dad.

I'm not sure how I can work in the whole force ghost thing, but I actually like this interpretation enough that I'm willing to bend a few facts to make them fit ;)