Kim Jong Nam, left, shown in 2001, was the exiled half-brother of North Korean leader Kim Jong Un. Kim Jong Nam was killed this month in Malaysia with a nerve agent that can cause death within minutes. (Shizuo Kambayashi, left; Wong Maye-E, right/Associated Press)

IF SPREAD on a battlefield or used against a population, the nerve agent VX would be terrifyingly deadly. A liter of the substance contains enough lethal doses, theoretically, to kill 1 million people. Less than 10 milligrams — a small drop on the skin — can kill a grown man. It has no other purpose than being an instrument of death. This is one reason most of the world has banned what is truly a weapon of mass destruction.

This is also one reason it is so monstrous that North Korean leader Kim Jong Un and his coterie of brutalists may have used VX to murder his half-brother Kim Jong Nam, while he was waiting to board a plane in an international airport in Malaysia. The killing, captured on surveillance video, displays the coldblooded mentality of Pyongyang’s young dictator, who has cut down relatives, colleagues and anyone else who might threaten him, while confining hundreds of thousands of people in bleak prison camps and building nuclear and missile forces to threaten the world.

North Korea often offers a bully’s proposition: just recognize the government and provide it with economic favors, and, perhaps, just maybe, it will not carry out the latest ugly threat it has manufactured. In truth, a deal with North Korea that ensures denuclearization once and for all would be desirable, if it could be achieved. But right now, Kim Jong Un needs to be shown there are penalties and no rewards for such gross violation of international norms.

The assassination by poison is reminiscent of how former KGB officer and dissident Alexander Litvinenko was silenced in London with radioactive polonium put into his tea. A British investigation found that the Russian security service and President Vladi­mir Putin “probably approved” it, although there was no direct proof, just a telltale radioactive trace left by the Russians who carried it out. In the same way, the killing of Kim Jong Nam may not have carried Kim Jong Un’s personal signature, but seems likely to be the result of his orders.

North Korea has had a chemical weapons program since the 1980s. It is one of the few nations that have not signed the Chemical Weapons Convention, which bans VX. Aside from the bucking of international norms, the assassination underlines grave questions about how Mr. Kim might behave in moments of crisis with the regime’s nuclear weapons. He is impetuous, irrational, bullying and armed.

North Korea ought to be placed back on the U.S. list of nations that sponsor terrorism, from which it was removed almost a decade ago. China made a positive move recently in curtailing coal imports from North Korea , but sanctions could be intensified still further against Pyongyang’s financial system and the Chinese companies that do business there. There must be no ambiguity in the message from the United States and its allies: This was outlaw behavior.