Rob was there. Mary stayed close to him once he'd pulled her out of the way. Regardless of the convoluted way they were related, he was blood. That meant something to her. And he felt like a brother. When the serial killer started shooting at her mom, Mary's first reaction was to try to get up and fight back. Something inside her snapped, even if her body was slow to react and wouldn't cooperate with the need for revenge.
Then her dad was there and he'd shot the woman in the knee. Mary watched as the woman went down. The killer swore, but didn't let go of her gun just yet. She pointed it up at Dean now. Mary bit her tongue to keep from crying out a warning. Her dad knew what was going on and she had to trust that he knew what he was doing.