North Korea wanted to hide their relation to the Aum cult, which was why they shot the Japanese police director general as a disturbance. They should have controlled the investigation with a help from their sympathizers in the Japanese police, though the director general, Takaji Kunimatsu, was not one of the sympathizers. It was a well-known fact that he actually claimed to investigate the crimes related to North Korea after assuming his position.
In any way, he was shot eventually and hospitalized under the coma. It was the time when the full investigation against the Aum cult was conducted, several days after their terrorist attack with the sarin against the Tokyo metropolitan transportation system. He had been hospitalized for months and could not command the most critical investigation at that time.
He was back to the office later on, but it was less likely in charge of an investigation of the shooting case of himself, but he had a total command hence it was necessary for North Korea to contain this investigation not going toward the real shooter of theirs by asking a support from the police insiders.
That was why they left their people’s army badge at the scene and sent a message to their contacts inside the police. This was a general secret code for the intelligence community, though they should have negotiated with the police contact beforehand, which was likely failed; that was why they must have sent a message in this way.
They wanted to show their determination to the high rankers who was difficult for the North Korean spy to directly negotiate with. Generally speaking, each spy should contact with their counterpart in the lower level, whose intel or request was transferred to the higher level, though there were many officers to control this information flow for their favor through the process.
I assume that information control really happened, which was why they shot the director general to contain the investigation reaching to their crimes and left the badge as a message to the sympathizer inside the Japanese police department. They immediately understood a meaning of the code and decided to cover up the real shooter of this case as well.
That was why this case was originally handled by the investigation division of the Tokyo metropolitan police, but switched to the police intelligence, several hours later of this shooting. This decision was made by one of the main intelligence officer, who was a head of this exact investigation division at that time.