[Looks relieved that she should understand the significance of him saying as much, and accept it.]
[He nods a little to her words, not denying it] And...
... I know that he would lay down his life for the two of you, without hesitation. And he'd do so gladly. [Because he's a fool and he loves you two]
That's why I'll protect him, in turn. No harm will come to him while I'm beside him, Your Majesty. I can promise that. [So you don't have to worry about him - although he knows she likely would, anyway, because it's Aramis and because she loves him.]
[He knows it's likely never going to come to that, in this place, but he knows it'd be the same back home, too: better his life than Aramis'.]