We share God's justice in a direct way in our very nature. No external learning is needed.
God's justice comes from God's nature and it's independent from God's will. God's nature is the source of all necessary truths.
I'm guilty because I'm not being myself. Because I live against my human nature which is an image of God's nature. Human nature are temporarily broken (but not totally broken).
But justice can only be neutralized by mercy. It's a zero-sum equation.
Mercy is a free act and is dependent on God's will.
So God Himself provided infinite mercy by being crucified.
But mercy being a free act (coming from God's will and not from God's nature) it demands a free act of acceptance too.
God's nature "punishes" (actually is an automatic/necessary thing) me but the God's will forgives me. My will only need to accept that mercy.
Please read more about molinism and you will understand much more about the Christian concept of God.
Sometimes I think people here only know the (awful) JW concept of God.