They both were calling for honesty and this was the most honest thing between them. Once they got past this, maybe they could heal the rift that had been made between them over the years. He knows he was such a dick to her, and she didn't deserve that from an older brother. She deserved someone better. From their world Harrington comes to mind. Kaplan comes to mind here in the city afterwards. A better Billy for her to call a brother.

If he feels her tears in his shirt, Billy doesn't say anything. He's got his own silent tears streaming down his face. They're both scared to die then. Billy is terrified of facing down that monster again in the mall. Especially knowing that there's nothing he can do to stop it.

"I'm gonna be better. I promise, Max." It's said so quietly, like he doesn't want anyone else to hear him even if it's just the two of them.