——The Bible clearly supports slavery ——
You are conflating two issues, but you know that...
Because of the new faith of believers in Christ, slavery became morally reprehensible over time...especially after the reformation when GODS word was no longer hidden from the masses.
Justice metted out taking another innocent life has always been seen as moral...
That you don't like the comparison doesn't make it unfair. If the standard for allowing an institution is whether or not the Bible allows it, then you are going to have to consider everything the Bible allows.