The Soul Thief (Angel of Death Series #1) by Majanka Verstraete

Goodreads Synopsis
When sixteen-year-old Riley is injured in a car crash and sees a girl stealing a boy’s soul, she’s convinced she’s hallucinating. But when she sees the same girl at the hospital later, she knows she wasn’t dreaming. That’s when Riley learns her secret heritage and who she really is: a halfling Angel of Death.

Riley must come to terms with her new reality and supernatural abilities, but before she can do this, girls her age start dying in mysterious circumstances. It’s up to Riley to figure out why, what the innocent victims have in common, and what she can do to stop them.