Cruelty doesn't exist where self-preservation is involved.
 F. Scott Fitzgerald, Tales of the Jazz Age (1922). copy citation

Context

“You're not a humanitarian and you're not fair-minded, but you're human—at least you say you are—and you ought to be able to put yourself in our place for long enough to think how—how—how—" "How what?" demanded Washington, coldly. "—how unnecessary—" "Not to me." "Well—how cruel—" "We've covered that. Cruelty doesn't exist where self-preservation is involved. You've been soldiers; you know that. Try another." "Well, then, how stupid." "There," admitted Washington, "I grant you that. But try to think of an alternative. I've offered to have all or any of you painlessly executed if you wish.” source