Justice Alito, joined by Justice Thomas, agreed that the judgment should be vacated but would have stated more directly that Massachusetts' categorical stun-gun ban violated the Second Amendment. In his view, the majority correctly identified the state court's conflicts with Heller, but the existing record already established the constitutional answer rather than merely requiring reconsideration on remand.
He emphasized that stun guns are plainly bearable arms under Heller's definition: weapons a person carries or holds for offensive or defensive action. Their nonexistence in the eighteenth century was irrelevant, just as the modern origins of electronic communications do not remove them from First Amendment protection.
Justice Alito also explained that the dangerous-and-unusual exception is conjunctive. A weapon cannot be banned merely because it is dangerous, particularly where it belongs to a class of arms commonly used for lawful purposes. Massachusetts' definition of a dangerous weapon was especially overbroad because it would encompass firearms, even though Heller held that firearms cannot be categorically prohibited on that ground.
On unusualness, he maintained that the proper question is whether law-abiding citizens commonly possess stun guns for lawful purposes today. The fact that stun guns are less prevalent than handguns did not make them unusual; hundreds of thousands had been sold to private citizens, and they were lawful in most States. Nor could Massachusetts rely on an alleged lack of military utility, because Heller rejected military usefulness as a condition of protection, and electrical weapons in fact had law-enforcement and military applications.
Justice Alito grounded this conclusion in self-defense. Caetano used a nonlethal weapon to deter an abusive former partner after restraining orders had failed. He rejected the suggestion that she could simply have carried a firearm or pepper spray: the availability of another weapon does not excuse a ban on a protected arm, and the State should not force a person who is unwilling to use deadly force to choose between self-defense and conscience.