MC didn't kill the cops, a few chapters ago we had a look at the police situation, they though that MC escaped the military base after the attack and the beheaded cops were caused by him.
He decapitated the bully, destroyed a police van with people inside, attacked the army at the cemetary, fought the mercenaries in the forest and the private guards and mafia in the hospital, they know he have no problem killing people in armed organizations, the mistake is that everytime he responded a attack.
They shouldn't know how the soldiers he killed/injured in the US base as they blocked all informations.
It seems like the girl have got some clarovayance power, maybe like the MC it simply came to her.
Girl was most likely violated by the bullies and developed a mental trauma, the bullies families probably made so she was put in a mental hospital where she can give no contact, maybe her family was paid or threatened, or decided to just shut up looking at what happend to MC.
Is even possible she is perfectly sane and is in the hospital for the bullies parents not wanting another scandal.
Question is who killed the cops, maybe it someone of the irregular squad?