"I gonna punch you in the face so hard you'll be #!@%ing fists!"
I'm hearing this said in my head in a southern American country-hick stereotype drawl.
As an opening line, it sets the scene quite well for violence and gives some fairly colourful personality behind the speaker and has a slight hint at humour if you find these kind of elaborate insult/threats amusing like I do.
I see the line being said to the protagonist, rather than by them and along that train of thought I imagine the protagonist responding defensively rather aggressively, trying to calm the person down. What did our protagonist do to warrant such a threat? Did they steal something from our antagonist, insult them in some way prior to the opening scene?
Alternatively, I quite like the idea of the protagonist responding in a wise-ass kind of way, a sort of "Surely I'd just be #!@%ing just one fist, unless you plan on punching me with both hands, in which case you might as well just dive in my mouth you inbred retard."