Sexual games gone wrong

My girlfriend and i like to play games when we're having sex.

What i mean by that is, her fantasy is to pretend I'm her sex slave and mine is to pretend she's my rape victim.
I know, it sounds messed up but we all have weird fantasies.

She'll usually tie me to the bed and do or force me to do whatever she wants.
And whenever i feel horny, i just "rape" her.
Of course this is all consensual, if she or I doesn't feel like playing, the game stops immediately.

Anyway, the problem is, she's really starting to take her role as a "sex master" seriously.
While I'm tied up to the bed, sometimes she'll bring a knife and to try scaring me.
She's a great actress which makes the experience even more terrifying.
Here's an example of what she would say while playing with the knife : "I went through your browser history and found a few porn sites. Do you think jacking off to other girls is fair to me? Maybe I should use this knife on you.."

I'm really starting to wonder whether i should keep trusting her or not.
While my fantasy is harmless, hers is dangerous.
If she decided to torture me with a knife while I'm tied up, I couldn't do shit about it.

What should i do?