Derek is determined to find out the identity of the Kanima and goes to extreme lengths to test who it is. He initially rules out Jackson and sets his pack on seeing if Lydia is the Kanima.
Derek falsely believes that a Kanima will not be hurt by its own poison, and after his betas test Lydia he believes he has found his Kanima.
To protect Lydia, Jackson, Allison, and Stiles all bring her to Scott’s house while he tries to buy some time from Derek, but Derek beats him to his house. Everyone fights for their lives, and it’s revealed that Jackson is the Kanima.