Mercy is not weakness when it is chosen with full awareness of the harm.

Leniency chosen from avoidance is weakness, and it usually shows: nothing was named, nothing was decided, the discomfort was simply skipped. Leniency chosen with full sight of what was done is a different act altogether. It requires knowing the cost, being able to impose a consequence, and deciding not to. That takes more strength than the consequence would have.

If you choose to be merciful, first make sure you have actually looked. Say to yourself precisely what happened and what it cost. Then, if you still choose to let it go, you will be able to hold that choice without resentment later, which is how you tell the two apart.