Am I sinful for having prevailed over a colleague who had previously wronged me, resulting in his dismissal, after he initiated a false complaint against me with the police?

Repelling injustice is Islamically permissible, as Allah Almighty says: "And those who, when tyranny strikes them, they defend themselves." However, forgiveness is better, as Allah Almighty says: "The recompense of an evil is an evil like thereof. But whoever forgives and makes reconciliation, his reward is due from Allah." Therefore, if you defend yourself, there is no blame upon you. And the dismissal of the employee is not your decision or a punishment from you, so there is no sin upon you.