If Kenobi had killed Luke's father (let's say because he was turning to the Dark Side like Vader) then maybe Luke would have felt more conflicted about sides--Vader would have been a stronger character(he could have still been a stronger character in ROTJ if he was competing for Luke with the Emperor).

Luke didnt really have much reason to be tempted by the Dark Side in ROTJ. The sister temptation comment by Vader was rather late in the game--it worked but not very suspenseful. The love triangle would have been stronger perhaps.

It would have been even more shocking if Vader and said "No, you killed your father."

Really should have checked all those detention cells on the Death Star before blowing it up.

;)