Malaysian police are assembling evidence that Pyongyang is responsible for last week’s chilling airport murder of Kim Jong Nam, the estranged older brother of North Korean dictator Kim Jong Un. This is one more reason the U.S. should redesignate North Korea a state sponsor of terror, a status it never should have lost in 2008.

According to investigators and video from the scene, the attack was carried out by two women, from Indonesia and Vietnam, who had practiced in local shopping malls under the direction of several North...