Mystery at Mesa Flat

A small desert town didn't seem a likely place to encounter murder—especially one that had been planned on a world light years away!
The murder was committed ten minutes before the Otarkian ship lifted for the long trip back to the mother-planet. It was discovered ten minutes after blast off. The killer—a great lout of an upper-hillman, signed on the last moment to fill a sudden vacancy—bragged of the kill to his sergeant.
Bragged grinning. He was crouched behind a rock peeking out at the ship. I came behind him—very quiet. I broke his neck and—and did other things. He never knew what happened.
The guard was rushed immediately before the Commander—into the dread Presence. The Commander's eyes were terrible but his voice remained soft. You know by what a slim thread our invasion plans hang?
Yes, Commander.
You know that utter secrecy has been our key from the start?
Yes, Commander.
I just wanted to make sure before I execute you in the name of the Supreme Otarkian Council.
Yes, Commander.
The Commander drew his gun and aimed accurately. The guard died bravely.
And that was that.
But there was worry. The Commander consulted with the Second. It would be wise to return.