He did not kill surrendering enemies. That is the code of the knight. But by Ami's order, he restrains him to listen to him as soon as she gives it. In spite of everything he felt -what every instinct dictated him to do-, he let him speak. Even the things of Metalia.

And when Mercury tells him their first concern, he lowers his head. Even if she hadn't given the order, he would have come for her.

When Ami gives him the following orders, of killing Kanda the second he so much as hinted about betraying their group yet again, he slowly nods. He is only to let him go if Mercury orders him so. Committing unnecessary cruelty was unbecoming, fitting more of barbarians than of knights.

And yet, this threat hurt Cortana.