I think this'd be a less cruel way to kill the stray population. If they don't act like jerks, but they have to go, just feed them something that's poisoned, and would kill them painlessly in a couple of minutes.
Considering what's left of the reimu looks like it was turned inside out in the background, whatever marisa is about to eat will not be painless. Heck it would probably be quicker just for chen to hold its attention while youmu rams the sword into its back side.