As they Intended

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The Mishna teaches us that the zomemin witnesses are only punished if they attempted to have someone executed, but they were found to be zomemin before the defendant was executed (as long as it was after the verdict was handed down). However, if they were discredited through hazamah only after the defendant had been executed, they will not be punished. This is derived from the Scriptural verse: as they intended to do; but not as they actually accomplished.

The Kesef Mishnah explains this seemingly perplexing halachah in two manners:
1. When the zomemin witnesses actually carry out their plan and the accused is executed - such a sin is of such a magnitude that they cannot get punished in this world. The punishment for such a hideous sin can only take place in the next world- in Gehinnom.
2. Alternatively, he explains, if the accused was actually executed, we assume that he was indeed guilty and deserved to die. Hashem is present by every court case and it must be attributed to Divine Providence that the second set of witnesses did not arrive until after the defendant was executed.