Well, I think Kitsune is somehow tied into Tang Shen. The flashback thing Leo had when he looked at her seemed indicative of something more, plus there's the fact that she claims her ritual will only maintain its effect so long as he doesn't get blood on his blade. If she is actually Tang Shen reincarnated in Kitsune's body, or something to that effect, it would make total sense that she doesn't want to send her son out to kill innocent people, or anyone else for that matter. With that theory in mind, these pictures could actually represent Leonardo being protected by his mother. What appears to be darkness (black mask) is actually light (white mask of his 'mother').
Wasn't tang shen contemporary to kitsune? I could see Kitsune working as a 'replacement' for his mother in Leo's mind, but that's about it. Btw, it took me a couple seconds but I also suspected the mask was a reference to kitsune's. Great thinking there.