It's an interesting choice to redeem Missy, as opposed to just killing her or keeping her as the recurring villain like they've done with the Master.

In reality, psychopaths and sociopaths don't tend to be redeemable, mainly because they don't possess the capability. (ie. They don't have a conscience, so can't feel remorse.) Example? basically every serial killer out there. And hit-men. I met one when I was working as a criminal defense attorney in Kansas -- interning with Kansas Defender Project. He had no remorse. Didn't care that he killed anyone. While I bank robber that I defended, did have remorse, and cared -- he was redeemable and not a sociopath.

So no, not everyone can be redeemed necessarily. Charles Manson never regretted his actions. Jeffrey Dahmer didn't. Ted Bundy, seemed to, since he asked for the death penalty and said he'd do it again, the compulsion was always there.