"You're supposed to be human" - OK fair enough, but what does that entail?

In one of the responses here to a question of if we are supposed to feel sympathy for Poncelet or if we are to think he should die and we shall all be grateful for it, one of the responders answered with "you're supposed to be human" but put a full stop afterwards and didn't expand or explain.

And OK I do agree with you. But what does that mean, especially in this case? To be human? To think that Poncelet deserves to die for what he did or to think that perhaps, while he may deserve punishment including many years if not lifetime in prison, that even the killing of someone like him is STILL wrong? And how does either position make one human?

Also, we in life are often told and taught certain things. But we've had some otherwise good people at times speak very ill of criminal perpetrators like him and think that by rights yes they do deserve to die and that no sympathy should be shed. Others rightly spoke of what we can or cannot do. But then, are there no grey areas or exceptional circumstances in life and are we, as human beings, sometimes right?

And who is RIGHT here, overall? And basically, being a human in this case means what? And how does one balance the desire for vengeance and that of the right punishment?

Forgiving criminals and letting them go is bad. Killing them or being too harsh is also bad. So where and how do we find this balance to be human? Inquiring minds want to know. :)

P.S. And was the Sister one and was she fully right? Thanks. :)