The Last Girl by Jane Casey

The teenage girl was the first victim.
Her throat cut to the bone, she didn't stand a chance.
Her mother was the second.
She, at least, had time to fight back. Briefly.
Called to the South London crime scene, Maeve Kerrigan's first thought is that this is a domestic dispute gone bad. But the husband - found bloody and unconscious in an upstairs room - insists he's the third victim not the killer. However, the only witness is his young daughter. And she's not talking...