XIETIAN
My guess is that Kat was with Liam, saw proof that Liam is giving the demon lord information, and he had to kill her to make sure it didn't get out. There's several things pointing to Liam being the traitor- BZ's weird question, the fact it has to be someone from the Gales kingdom that doesn't know he is from earth (and is still alive), but is strong enough that they could communicate without being discovered