I see your logic and and its appeal but it is flawed, you might like to consider what Jesus said:
Matthew 21:25-34 (NIV) The Parable of the Two Sons
28 "What do you think? There was a man who had two sons. He went to the first and said, 'Son, go and work today in the vineyard.' 29" 'I will not,' he answered, but later he changed his mind and went. 30. "Then the father went to the other son and said the same thing. He answered, 'I will, sir,' but he did not go. 31. "Which of the two did what his father wanted?" "The first," they answered.
Jesus said to them, "I tell you the truth, the tax collectors and the prostitutes are entering the kingdom of God ahead of you. 32. For John came to you to show you the way of righteousness, and you did not believe him, but the tax collectors and the prostitutes did.
One might argue that the the one who changed his mind eventually had the right intention but might have done it out of duty not desire for good. So the good act is still good surely? Alternatively, one might see the one who changed his mind as confessing his wrong doing and following it through with repentance and action to show it was real.
The other point here that you seem to be missing is that I can see God judging our good deeds but surely he must also judge our bad ones, in other words he judges all our actions. But is it not startling that Jesus says the worst of society will get into heaven first, not because of their good deeds but because they believed in a way of righteousness enabled by faith demonstrated by repentance.
Finally, if I take your point about Jesus then by extension it means that it is always unjust to take another persons place. So if I dive into a river to save someone, even someone who society regards as worthless, risking my life in the process that according to you is unjust. Indeed any selfless act is by the same token unjust.
Here we are back were we started by your own argument, if God forgives a sinner it is unjust, the sinner rightly should be punished. No one would regard it as just to let the guilty go free. Ipso facto according to you God would be unjust to show mercy.