Disclaimer: I don't own Glee or anything associated with Glee. Poor me.
Brittany explained to Quinn that Santana had 'episodes', and during these 'episodes' she felt urges. Urges that weren't completely normal, and definitely weren't completely human (at least if the glowing yellow eyes were anything to go by). Santana had described them to Brittany once. A feeling deep within her chest that seemed to curl and clench at her heart and mind. Santana had told Brittany this after the first time.
They had been seven and had run away from Brittany's parents in the mall to play hide and seek. They eventually settled down in the less used car park to be found. But it wasn't Brittany's parents or older sister that found them; it was a man they had seen on TV. A man who was being tracked down by the police for the kidnapping, abuse and murder of three girls. He had grabbed Brittany around the neck and was reaching for Santana when a jagged stone was flung into his face with more force than a seven year old could manage. He dropped Brittany and stumbled back. Santana shoved him, and he stumbled again, falling over. He turned over, trying to get up, but Santana grabbed him by the hair and smashed his head into the ground. Once, twice, three times. Brittany started to lose count. Once the man stopped struggling, Santana let go of his bloody head and stood up. Some of his hair had stuck to her hand. She turned to face Brittany, and Brittany knew that she should be scared. What with her best friend having glowing yellow eyes instead of her normal warm brown. And having just killed a man, but she wasn't. She rushed forward and engulfed Santana in a hug. Brittany knew the man was a bad man (her mum had told her that) and that Santana had just saved her from being hurt.
"Thank you."
There had been a few other times, but Brittany made it clear that all those times had been justified. Either as self defense, or as defending someone else. But every single time, Brittany had been there to hug Santana and calm her down. Quinn had no idea how so many bodies (in lima even one murdered person was a lot) hadn't been found, but they hadn't. Quinn was sure she'd have heard about if they had. But she knew she couldn't tell anyone. Santana had saved her. She wasn't naive enough to think that nothing would have happened with Tom after he dragged her upstairs. Quinn knew exactly what would have happened, and it was a scary thought.