If done, dare I say it?, correctly, then the prequels would lead you to believe that Obi-Wan had, in fact, killed Luke's father. If Episode 3 ended with the Obi-Wan vs Anakin duel which results in Anakin's apparent death, then the viewer would leave the theatre thinking Anakin was dead at Obi-Wan's hand.

Episode 4 starts, and we get introduced to Vader whom we do NOT immediately know or suspect to be a remade Anakin. Obi-Wan tells Luke that this Vader killed his father, but we know better. We trust that Obi-Wan is a good guy and all, but we find it suspicious that he's lying to Luke right off the bat, hiding his own guilt in the matter. I know a lot of people don't like the "Obi-Wan the Liar" mechanic that starts at the end of ESB is developed further in RotJ, but I don't really mind it. In fact, I would like the intrigue I just referrenced above. Then, in ESB, when Vader tells Luke, "Obi-Wan never told you what happened to your father" the naive viewer thinks: "Uh-oh, here comes the truth about Obi-Wan actually killing Anakin, and that Vader had nothing to do with it!" And then you're hit with an unexpected twist: "No, Luke. I am your father!"

So yeah... Obi-Wan killed Luke's father. Or at least he should have.