In 5E? I'd let you do it if you could kill him in one round of attacks. (IE: Have you do all your attacks, if it kills them, then sure, flavor it as as ripping his head off.) I wouldn't allow you to just do it with a strength check. I'd give you advantage on your attacks (and probably take away his dex bonus to ac) since he's unaware of you, but I don't think anything past that.
I have a +11 to damage right now, and I use a greataxe. I've dropped dropped most enemies thus far with my teeth being a longtooth shifter, and even successfully made a hardened dwarf soldier walk up a corner, look at me mauling his buddy with my teeth, immediatley turn right around and walk the other way. The other soldier watched me bite the guy through a window and pull him out and heard the screaming, walked up to me and because my DM knows they have never seen a shifter (or a werewolf, which I said with the flavor and the fact of backstory, I'm a pure-blooded werewolf that was cured of his disease, so I am considered a shifter) and I proceeded to make that one try and stab at me but he missed and proceeded to vomit. When I screamed at him, he ran away, and the other dwarf who noped, jumped into the building, smashed through the door and bumped into said fleeing dwarf who was running from me and then shits his pants and passes out.
The one who was awake was wounded slightly by my teeth earlier, we questioned him, gave him food, and kicked him out into the snow to warn the leader of opposing side of the civil war we were on:
The White Fangs were coming for him.
So yes, I essentially made my party the inglorious basterds