The fact that a line could be drawn badly is not a great reason to not draw the line well. People who didn’t commit murder can also be convicted of and punished for murder, but that’s not a great slippery slope argument for never punishing murderers.

It's in fact a great argument against death penalty for murder, and such argument is commonly made.

That’s only an argument against the death penalty if accompanied by another argument for why the death penalty is distinct from other forms of punishment, otherwise we would conclude that all legal penalties are bad for the same reasons.