You don't always turn into a wolf if you're bitten. You turn into a reflection of who you are inside. I think. Maybe. Anyway... murder lizard, what's known as a Kanima, didn't know his own identity either. Derek said a Kanima was immune to its own venom (NOT a fun experience, let me tell you). Lydia was immune, still don't know why.
So, Derek sent us to kill her because the Kanima was killing people in town.
TLDR; Lydia was innocent so we didn't kill her.
And yeah. Mean-Girl popular. And stupid smart. Worst combination ever.