If you talked with Lilly, she'll say Larry, her dad, is the only family she has left. He might be an asshole, I think he was on steroids, but he's just looking out for his daughter.
And then Kenny, who was too scared to help Shawn, is god-damn sure that Larry was dead inside that locker. Then smashes Larry's head with a salt-brick.
Think about it, if you were Lilly, wouldn't that make you crazy?