Second Contact — Mike Resnick

On board the starship Theodore Roosevelt, in deep space:

Commander Wilbur H. Jennings, a career Navy officer in command of the ship, walked out of his cabin one morning and shot two crew members to death. Then he confined himself to his quarters and turned over command of the Roosevelt to his executive officer, suggesting strongly that the ship return to base immediately.

The Pentagon, Washington D.C., United States, Earth:

Major Max Becker has been called back from leave and handed the job of defending Jennings at his court-martial. Becker's superiors will accept a plea of temporary insanity and even provide the witnesses to prove it. The trial is to be brief, no death penalty will be invoked, and I public outcry will be stilled. Becker isn't happy about what the case will do to his reputation, but at least it isn't complicated.

There's just one hitch.

Jennings refuses to plead insanity of any sort. He maintains that he was justified in killing the crewmen. They were not human. They were alien spies.