If the Nutcracker wishes to employ more subtle means of exterminating the Mouse King, he may be well advised first to consult FIFRA (the federal Insecticide, Fungicide, and Rodenticide Act, at 7 U.S.C. §136 et seq.), the statute that regulates which pesticides are available for poisoning mice and other "pests". FIFRA imposes registration, labeling, and application restrictions supervised by the Environmental Protection Agency. However, stabbing with a toy sword is probably outside the scope of FIFRA
If the particular species of mouse attacked by the Nutcracker happened to be federally listed as endangered under §4 of the ESA (the federal Endangered Species Act, at 16 U.S.C. §1531 et seq.), then anyone harming that rat might risk committing a "taking" under §9, and thereby be subject to civil and criminal penalties under §11. For example, production of The Nutcracker in Southwest Florida that tapped local mouse talent might be advised to exercise caution if its Mouse King were a Key Largo Cotton Mouse (Peromyscus gossypinus allapaticola), a species the Fish and Wildlife Service officially lists as endangered. In defense of the Nutcracker, his taking, done, as it was, in defense of Clara, might fall within the exemptions or exceptions provided for in §10.
In addition, a growing web of animal welfare laws at the federal, state, and local levels, not to mention ever strengthening norms against animal cruelty, might give the Nutcracker pause, perhaps convincing him to call in the local animal control agents rather than resort to deadly self-help. Finally, the Nutcracker should probably verify whether or not the state in which he uses deadly force against the Mouse King imposes a duty of retreat, even in his own home.