Justice can be mercy because putting an evildoer out of their misery accomplishes both - justice in giving them death and mercy in saving them from self-inflicted suffering. Mercy can be justice because in forgiving us of our sins to prevent our death we can receive Holy Spirit's help to become righteous, allowing us to become worthy of the life that was gifted to us.While justice and mercy are both employed by God, I fail to see how the two are applied simultaneously to the same individual. Mercy requires God to relent from doing justice, and not apply to the individual the justice their actions should require of them. God can remain just in doing so because in Christ He has already exacted justice. But I don't see how both justice and mercy can occur simultaneously.
I am still exploring this idea myself, but I don't see them as necessarily being in conflict.