There's nothing inherently wrong with the gradual progression from good to evil you mentioned. But that's never actually how it's played in DE.

Han is skeptical of Luke's intentions, but Leia vouches for him. She senses his "sacrifice" in the Force, and believes him to still be a Jedi at heart. They end up taking down the Devastators on Calamari (a major blow to the Empire), which wins the reader's trust. Luke is only really tempted by the dark side near the tail end.

I'll set aside my criticisms of ROTJ for a minute to say that the dark side, in my opinion, is scarier when it dominates suddenly. Luke came damn close there to taking his father's place at Palpatine's side, because he lashed out in anger. If he'd wanted for a molecule less of self control, he would have gone from hero to villain in seconds. That's more disturbing to me than the other method.