There was the slightest tinge of disappointment when she said that she hadn't been bitten. He wouldn't have minded having a gun, even though he imagined it would take some time to get a hang of using one. Still, he wasn't dropping his weapon until she was. She may have had a gun, but she didn't look in the best condition. On that note...
"Because you look like one of them!" Even though she might have been oblivious to her current state, he certainly was not. He didn't need a better look to tell her that much. And he didn't feel at all bad about telling her either. "And I'm not talking about your stupid sweater, you're limping around moaning and bleeding all over the place. Actually I've seen zombies that looked better!" Which may have been a lie, if not an exaggeration, but he was defending a point.