I guess since this is an internet forum your taking my legitimate question as being a "gotcha", but whatever it's all love on my side. If you reduce punishment to just inflicting harm for emotional retribution then yea of course it's going to be unjustifiable, but if your going to be as charitable as possible to the crowd who supports punishment, then you should consider the advocates who support things like comparatively "painless" executions in which the suffering is minimized. However, the advocates of punishment are to my knowledge not specifically defending capital punishment, but punishment in general, which would seem to include something like imprisonment which I'd assume you'd support over the execution of war criminals.

Overall, I think I tend to agree with what your saying as I don't think its right to cause unnecessary suffering out of some sense of justice, nor am I supportive of executions when one can be rendered powerless by other less harmful means. However, that doesn't stop me from thinking that your notion of punishment seems narrow, and while the critique your making may be applicable to certain forms of punishment, that it fails when considering less extreme forms of punishment which don't aim to bring about unnecessary harm unto the offender, but to merely render them harmless.