That takes a LOT of faith, to remain armed in the vicinity of a rampaging gunman, and an approaching police siren. (But then I repeat myself)
Even odds on the cop shifting aim from the covered target, to Stacy, as Stacy begins to fire at the gunman. Especially if Stacey is an easier shot to make.
I'd probably engage a looney on my block if it ever happened, but I'd get hidden and gone quick if it wasn't resolved before I heard sirens.