There is the wise saying: "One who fights and runs away, may live to fight another day".
Today I came up with something that's closely related, but perhaps with a little more Victorian-style dash(?^), and it goes: "Vengeance is a luxury the dead cannot afford".
Imagine that line spoken by the head of a small clan that's just seen its territory overrun by a dastardly enemy, and whose people are torn between two cruel choices: mass suicide, or being taken into cruel slavery. I think it would be a nice fit in a novel by Dumas or Sabatini^.
IN THE REAL WORLD, COWARDICE IS SURVIVAL-ENHANCING
I suppose a more pithy, snappy version is possible for the internet age -- imagine Schwarzenegger's T-800 Terminator wisecracking "Dead humans can't shoot back" (pity that it's been reprogrammed not to kill humans).
Of course, that version carries a somewhat different meaning. It's gone from defiance to a kind of satisfied gloating -- the difference is even more obvious in this Mafia movie version: "A dead man can't get even". How about that^.