Is Jesus sacrifice on the cross an act of God's justice or an act of God's mercy?

The answer is "yes." Justice is served when Jesus died on the cross. Mercy was given to us because Jesus died on the cross.

God gives us mercy every day of our lives. That won't be true when we leave this world.

Our works will never earn us our salvation but they will definitely earn us our condemnation.

God is both the just and the justifier.