I'd refine the theory a bit. Grimm was drummed out for cowardice, not murder, so my guess is that the captain was killed by legitimate enemy action, the first officer was the coward, the three lieutenants (technically) mutinied and salvaged whatever mission they were on. The first officer (assuming he woke up) would have gone with it since accusing the lieutenants of mutiny would have revealed his cowardice.