If Lucas wanted a story about "trying to understand evil" and how "evil" can start out good, then Palpy must be given the same creds; therefore, Palpy must have a chance at redemption too. Geesh, if Vader can kill kids and be redeemed, why not Palpy?
Here's how:
If Obi could wait for years for Luke to grow up and have millions of death go on, why couldn't Obi wait another 10 years and have Luke truly master the force to a degree that's unheard of. If Luke was strong enough to beat the geriatric Vader and beat a near death Palpy with one finger, Palpy would have respected that... Cause Palpy only respects power. Luke would then use the mind trick and, pardon the expression, mind rape Palpy and find out why he turned evil and change the evil of his ways. Palpy would cry and say how sorry he was, and then he could be a force ghost and stand next to Yoda at the end.