Thief backstab against a player in your own combat

I am a thief fighting a level 20 which bad stuff includes death. I am level 1 with a +1 item equipped. I ask for help and they accept.

Our combined is now 21. I have no cards in my hand so I backstab the player who is helping me by discarding my +1 bonus that was equipped. The intentions are to make the player helping me die along with me.

Is this legal?

Thief backstab against a player in your own combat