"MOTIVE. An idea, belief, or emotion that impels a person to act in accordance with that state of mind. Motive is usually used in connection with Criminal Law to explain why a person acted or refused to act in a certain way—for example, to support the prosecution's assertion that the accused committed the crime."
His partner that night has already given the motive: "they were both pretty spooked". But only one of them acted with a gun, fatally firing at the person who called the police.
He labeled his partner a spook?