Well strictly they are voting on whether there is a blood debt, not on the sentence. But you're right, at this point Harry will have to offer something else dramatic and taboo (like withdrawing a vow of enmity and swearing instead to be a minion to Draco or some such), just to get Lucius to budge at all, while still giving him the level of vengeance he claimed he originally wanted. So Lucius himself doesn't have to betray a "sacred" value (which he won't).

Another thing I noticed: Harry basically awards Lucius the idiot-ball for playing his appointed role in an obvious set-up. But isn't he doing exactly the same? (Internally declaring war on the wizarding world, not minding any more if anyone thinks he is a Dark Lord, vowing enmity against Lucius, then going to the dark side...). Not very rational.