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
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.