Fiance's friend hit on me...

This happened 6 months into our relationshipand I still haven't told him... I was sitting in my fiance's room with him, his brother and his friend. My fiance's brother got up to use the restroom, and my fiance left to go to the store, his friend comes back in from a phone call I'm sitting in the middle ofthe bed with my back against the headboard his friend sat next to me I looked at him like

Then he's all like "what's up October" I'm like... "nothing?"

Then he's grabs my arm in attempts to pull me closer to do idk what, I litterally jumped off the bed so quick then he starts apologizing all crazy like I'm sorry my bad sorry sorry

I never told my fiance because he's crazy... literally insane... like I'm confident he would have killed him which is why I have kept this from him so long but I also feel like we shouldn't have secrets so I asked him a "hypothetical" question I said "hey babe what would you do if one of your friends hit on me?" Hey said "haha I'd f*cking kill them and eat them."...... I'm sorry what?

But we have moved since then and he really doesn't talk to that person any more should I tell him or take it to the grave??????????