What leads up to Lana's death in Chapter 55? Who kills Lana?

Lana is battered, bruised, and bloody, and much of her hair has been ripped out. Three men surround Lana. One stabs her with a switchblade. Mark tackles the man and stabs him with his own switchblade. Alec tackles another attacker. More men begin to go after Lana. They shoot as many as they can, and then Alec shoots Lana, ending her misery.