Colossians 3:25 does not address human revenge only. It refers to the fact that God judges unrighteous behavior. God is paying attention to the unfair treatment you have been enduring. He will issue revenge on your behalf at some point. God will defend his children.
It might be difficult to understand this in the light of what he allowed Jesus to suffer at the hands of evil men. But that is only because we are looking at the finite picture. The judgment day will be a frightfully catastrophic day for those who practiced unrighteous behavior.
Every scoffer, every murderer, every gossip, every vengeful person, every unjust person, everyone who rejected Christ will stand before the judge of all. Though difficult at times, choosing the benefits of God’s grace far outweighs the cost (Hebrews 10:31).
It is incentive for those of us who are the employer to remember the words of the Holy Spirit, “Bosses, give your employees what is just and fair, knowing that you also have a employer in heaven” (Colossians 4:1 paraphrased).