The Female of the Species by Mindy McGinnis
Synopsis from GoodReads:
Alex Craft knows how to kill someone. And she doesn’t feel bad about it. When her older sister, Anna, was murdered three years ago and the killer walked free, Alex uncaged the language she knows best. The language of violence.
While her crime goes unpunished, Alex knows she can’t be trusted among other people, even in her small hometown. She relegates herself to the shadows, a girl who goes unseen in plain sight, unremarkable in the high school hallways.
But Jack Fisher sees her. He’s the guy all other guys want to be: the star athlete gunning for valedictorian with the prom queen on his arm. Guilt over the role he played the night Anna’s body was discovered hasn’t let him forget Alex over the years, and now her green eyes amid a constellation of freckles have his attention. He doesn’t want to only see Alex Craft; he wants to know her.
So does Peekay, the preacher’s kid, a girl whose identity is entangled with her dad’s job, though that does not stop her from knowing the taste of beer or missing the touch of her ex-boyfriend. When Peekay and Alex start working together at the animal shelter, a friendship forms and Alex’s protective nature extends to more than just the dogs and cats they care for.
Circumstances bring Alex, Jack, and Peekay together as their senior year unfolds. While partying one night, Alex’s darker nature breaks out, setting the teens on a collision course that will change their lives forever.