What I think is interesting about this is that Go aggro get's its weight from the fact that you have to deal harm if they suck it up - It's a promise of pain that you roll to prove that you're good for. You don't actually want to kill them - You want them to do something for you.
The quarantine, if s/he is too disciplined to shoot in cold blood, is not good for his promise of pain. In a way, his move is as much a handicap as a boon: Apocalypse World citizens might suck it up if they detect that this pre-apocalypse softie doesn't have the guts and hardened cynizism to kill them outright.
This is my read as well: if the Quarantine regularly Goes Aggro and then doesn't actually follow through, she may develop a reputation as a softie. Then again, lots of lesser-harm actions (like kneeing someone in the nuts) would certainly qualify as follow-through, even if they aren't a shotgun blast to the face. But there's only so many times someone can threaten to shoot your buddies with a shotgun and not do it before you stop taking their threats seriously.
And also, pursuant to Vincent, I prefer the 0-harm option to S-harm (which in my opinion is outside the purvey of the move, though I may not remember it properly.) If you clock somebody in the head with the butt of your rifle, but only want to do 0-harm instead of 1 or 2, then you're that much less likely to successfully knock them out.