Quackula said:

Obi-Wan doesn't necessarily have to be the villian to have killed luke's father.

Sure, you could've then dicked out and give it a context in a ROTJ-like, more lighthearted follow-up, but really, what DOES the "Obi-Wan killed your Father!" and subsequent shocked reaction really imply at first glance? And what kind of shock value does "they were in a space battle and Obi-Wan kinda blew him up while trying to fend off buzz droids" reall have?

Also there would be a kind of narrative problem with yet another backstory jedi having turned evil at the same time as Vader, and then the other one killed, along the lines of second death star.

But not saying either alternative would've been inherently bad, depends on how it would've been continued and wrapped up, obviously.