Didn't the Federation do away with the death penalty in Kirk's time? If I remember correctly, it only existed for travelling to Talos IV, and then only upon sentencing by a court martial. I cannot believe this same Federation would look kindly on the anti-Lex Luthor argument for extrajudicial execution. (It is noteworthy that no one ever tried that on Luthor, either.)
Nor can I believe that Data has been given free reign to abrogate the rights of any other Soong-type androids as some kind of "Soongian emperor." Unless he has been recognized as the sovereign of a state by the Federation, he has no more legal right to commit violence against any other Soong-type android than I do.