so. just to go back to the OP a bit. I watched that video a couple times and the attacker had the knife in his hand at the point when he walked up to the victim as far as I can tell. The victim didn't notice this.
Seems like a bit of an outlier case according to the below but definitely one never knows so better to de-escalate with people acting strange.
https://lawandcrime.com/crime/nyc-man-caught-driving-slain-doctors-mercedes-charged-with-multiple-murders-during-violent-spree-nypd/