In speculative fiction, mercy often risks reading as naïve or preachy unless the world pushes back.

By “mercy” I mean: sparing someone, helping an enemy, or refusing to punish when doing so is riskier than being ruthless.

What’s one technique you use to make mercy feel credible and tense (rule, cost, consequence, backlash, etc.)?
Bonus: any favourite examples where mercy worked, or where it killed the stakes?