I think most people maintain an illusion of themselves being infallible, and that any mistake they might make is only ever going to be harmless or innocent.

So the sticking point there isn't even that the punishment needs to fit the crime, even if they were the criminal, but rather the idea of them having to think of themselves as being in any way malicious, if only hypothetically.

Nevermind that there is malice in all humans, and that it is also possible to commit crime/evil without malice.

Most people who haven't contemplated the matter much, seem to knee-jerk themselves into two conflicting positions, at the same time.

They did evil, therefore they are evil, and deserve the worst thing I can imagine.

And

I am not evil, even if I do evil it will never be intentional, therefore I deserve no punishment and will never receive any. Any punishment I receive is undeserved and unfair.