From my perspective, it's because as a fellow human I can imagine his thoughts and feelings and in turn care about them that I believe he should be treated humanely. That imagining (and caring) is empathy. Where does your belief in humane treatment come from?
I suppose I set that as the baseline without needing to specify an appeal to innate humanity, so I agree.
It might be what enables you to treat the subject humanely in the first place. It seems it would certainly make it easier, at least. Advocating empathy isn't the same as being soft on crime. It's perfectly possible to punish for crimes, even with death, and still display empathy and compassion.