So, since God's mercy is effective, when he looks upon a sinner and makes a judgement of righteousness upon them, then he necessarily takes the responsibility for their sin upon himself. Just as if a human judge declared an obviously guilty person innocent and let him go free, would not the victim now accuse the judge of a crime? Would the judge not now be culpable for the crime of the person he had mercy on? With God His judgement it is actually effective, meaning if he judges you innocent you are innocent, and naturally, to all those you have victimized, your sin necessarily and totally now falls upon God.
So by God having mercy on a sinners, the victims of those sinners now blame God. But what are we going to do? Crucify Him for it?
There is a reason why Christ is the Judge of mankind, because he is also the one who is willing to take our sin upon himself, by means of his mercy. It is simple enough for Him to simply forgive our sins against Him, it is another thing all together for him to forgive our injustices against each other and for us to still be able to call him just. But as it is, He is both just, and the justifier of those who believe in Him.
The cost of our freedom is suffering, even for God.