Tell her the truth.
From Create Your Own Story
You tell her every detail of what lead you to the current moment. She listens, never interrupts, and never asks a question. When your done, she steps away from the door.
"We're not bringing her in here. She's one of them now."
"One of who?" You ask.
"The monsters. The freaks running around killing and fucking everything, pendejo. Are you stupid?" She points the gun at you. "You want, you can go back out there, but you ain't coming back in. When she gets enough strength, she'll fuck you up. Or, you can stay in here with me, locked in and safe. Your choice, but we ain't discussing it no further."