[Bells]

I think the difference is that those who plead guilty have come to terms with what they have done, rather than continuing to deny it or not accept the fact that they've done it. Recognising the fact that you have committed such a horrible act means there is more hope for remorse/repentance.

Those who plead guilty genuinely do (for the most part) feel guilty - whilst those who can stand indifferent have not realised the magnitude of their actions. There may well be exceptions, but generally murderers should at the very least come to terms with what has happened.

They don't see what they've done as an inhumane act, and are therefore not deserving of a humane punishment.