“As the decision that is not swiftly acted upon regarding an evil deed, the heart of men is set upon them to do evil.”
The idea of injustice can be approached in two ways. On one hand, there is injustice when the innocent are punished for a crime that they did not commit. On the other hand, if a punishment that ought to be given is not given then that too is unjust. The first instance is a personal injustice and the second is a societal injustice. In the first case, the unjustly accused suffers and in the second case, society as a whole suffers.
While Solomon recognizes the first case (and speaks to it often), the second case is where his focus is at the moment. When the law determines that such and such is an appropriate penalty for a crime and a decision to punish is made by the judge, but that decision is not acted upon, then an injustice is done and the law, that is designed to protect the people, brings harm to the people because it ceases to be a deterrent for sin for the broader community.
We all know that in our homes, when raising children, if you threaten a punishment but never act upon that punishment then the children will cease to obey. The same exists in the workforce. If the boss threatens to fire you unless you do “X” (whatever that might be), but he never fires anyone, then the employees will cease to listen to his threats and will do whatever they think is best. If church leadership threatens to excommunicate someone but never acts on the threat, people will not respond and the leadership will lose the respect of the congregation as a whole.
If sin is not acted upon by the powers that God has put into place in this world, then we tend to set our hearts on sin. So says Solomon; so says human experience.