The question of justice is not an "all-or-nothing" concept. Humans bring criminals to justice quite frequently. Arguably, the human system often goes overboard "bringing people to justice" for trivial infractions.

However, we can't guarantee we'll capture and punish everyone who deserves it. Murderers and rapists will get away, and some of them may live out the rest of their lives in peace, never to be brought to trial.

It is an imperfect world, after all. Many theists think they have the solution for this - God.

The main problem is that they can't demonstrate that any such "ultimate" justice even exists. At best it is a desperate hope to make them feel better about the fact that our human criminal justice system is not 100% effective.

Worse, such a mentality actually worsens our capacity, as humans, to enact justice.

It is called the bystander effect[1] - that people are less likely to offer help/aid to a problem, when they think others will take care of the problem. "Passing the buck" to God, to deal with matters of justice, is bound to have an individual less likely to help out in the real world, because an invisible sky wizard will intercede.

If the house across the street is on fire, do you personally grab a bucket of water and go help, or do you wait for the fire department to show up? If there's potholes in the road, do you personally fill them in, or wait for the road crews to take action? If a rapist escaped human justice, do you personally join the hunt, or wait for a supernatural detective track him/her down?

Ultimately, the only "justice" we know of, that actually can be demonstrated to exist, is weakened by those who think their god is going to take the helm. Whereas, realizing that we are on our own to solve our problems provides more pressure and impetus on us to actually get off our couches and do something to better our world.

If you know that you're the only person who can help with the burning house, you're much more likely to lend a hand. Our criminal justice system may be imperfect, but it is the only one we know we even have.