How old is her daughter?

just leave. don't stay. run.

yep not terrifying at all

I have a step dad , you shouldn't feel threatened , trust me she will threaten you , but she won't hurt you because by doing that she'll hurt her mom . Tell her mom what's going on and it will stop . in worst case scenarios ... she'll not believe you and maybe leave , but the little girl just feels threatened by you . She doesn't want you to replace her dad . I'd sit down and talk to her about that and let her know . unless you don't see yourself in that family in the future then gtfo because you're wasting their time . (:

Well that escalated quickly. On a side note that is a scary child

Run!

You are an adult. Go have an adult conversation with your girlfriend (not whining that her kid is being mean, but an actual conversation expressing concern over these words and what might actually be causing them). If your girlfriend is also an adult she will address it accordingly. If your girlfriend automatically calls you a liar, you might want to rethink the relationship.

Dump her if her kid scares you. Not a healthy relationship..

Talk to the girlfriend. If nothing changes, the kid isn't yours, so just break up with her and move on. Not all kids are psychos.

Time for an exorcism?