Alice in Murderland by Kaori Yuki

Every month, Stella and her eight siblings must attend a Mad Tea Party with their parents. This month, their mother kicks things off by announcing that the kids must kill each other until only one is left. To ensure their cooperation, she has taken hostage each child's most treasured person. When chaos erupts, Stella finds herself transformed into a blonde-haired, blue-eyed girl named Alice. Stella doesn't much like Alice, but her murderous alter ego is her only hope of survival.

The series is rated Older Teen by the publisher. It's grisly, with people being murdered in a creative variety of ways, and will appeal to fans of Deathnote and Tokyo Ghoul. May also appeal to teens who like the psychotic characters and rampant battles in the Alice in the Country of Hearts series, but this is not a romance.