[It's not that Nathan doesn't trust Kitty to avoid doing stupid things that people who were recently stabbed should stay away from, but... actually, he doesn't. That said, he's not particularly good at displaying concern in a readily recognizable way, which is why he is shadowing her while pretending that that's exactly what he isn't doing.
Why talk about worrisome things like Kitty going off and getting hurt when it's not absolutely necessary?]