It's not so much the mopey old git he turned out to be - that's perfectly reasonable considering how much of a whiny teenager he was. It's more that there wasn't enough time devoted to him being conflicted about murdering his nephew. Remembering the lengths he went through trying to turn his father back to the light side, I just didn't buy there was no other way than to murder Leia's son while he slept.
Mainly my issue is, that after forty years, that was the best story involving Luke they could come up with.