“I gonna punch you in the face so hard you’ll be shitting 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 shitting 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.”