She Killed Him First - American Injustice, #1

It's a smoky autumn morning when Kimber Clark gets the alert: her kindergartner's school is on lockdown. She's still drunk when she joins other parents outside the police tape, but not so drunk that she can't pick up details: Zack Linzy, a high schooler, opened fire in the school cafeteria, killing four people. But that's not possible. Zack Linzy is a freak, a boy who assaulted Kimber's daughter, and Kimber killed him with a shovel two days ago. The police are looking for the school shooter in the wrong place, and the real killer might get away with it.
Kimber can't tell the police they've got the wrong suspect without showing them where she buried the body. No-she has to solve a school shooting, absolve her daughter's abuser, and make sure the real killer comes to justice. All without getting caught for the murder she committed.