The trail leads them to Kate , the werewolf they let live in Season 8 ("Bitten").
After Tasha turns more people and refuses to stop killing, Kate realizes her sister is too far gone and kills Tasha herself. Key Character Moments Supernatural, Season 10, Episode 4: Paper Moon [S10E4] Paper Moon
Kate wants to rehabilitate her sister, while Dean believes they must kill her because she is a monster. The trail leads them to Kate , the
While the brothers suspect Kate, the true killer is her sister, Tasha . Kate turned Tasha to save her life after a fatal car accident, but Tasha has embraced her monstrous side. The trail leads them to Kate