Murray Leinster's "The Black Galaxy" (novel, space opera, free): Accidental human hero takes on murderous aliens
Story summary.The first human interstellar ship, Stellaris, still half finished, accidentally takes off due to a short circuit - with the hero & ship's designer, Rod Cantrell, on board with his girlfriend Kit. Plus some workers who didn't go out during lunch time.
They'll have major adventures hunting an alien race of murderous barbarians. Barbarians that wait for civilizations to mature, then come kill the whole world with a death ray & do the lootings!
Fact sheet.First published: 1949.
Download full text from Internet Archive.
Note: The 1954 novel version I've linked is either an expanded form of a novella of the same title that first appeared in Startling Stories, March 1949, or the same novella published as a novel. I'm not sure.
Related: Stories of Murray Leinster; Space opera fiction.