I liked the discussion at the end best. It actually made the episode for me. And changed my mind about Missy/Master -- where she states that she remembers the names and faces of everyone she's killed and there are so many...how he didn't tell her about this. Tears streaming down her face. He says how this is actually a good thing.
The writer's of Doctor Who are obviously not pro-death penalty and believe in redemption -- I agree with them for the most part. Death solves nothing, unless someone is a threat and there is no other way to stop it. But I'm not sure about redemption. I've met and seen people who...well...can a sociopath truly change?