"15 And thus he shall bring salvation to all those who shall believe on his name; this being the intent of this last sacrifice, to bring about the bowels of mercy, which overpowereth justice, and bringeth about means unto men that they may have faith unto repentance.
16 And thus mercy can satisfy the demands of justice, and encircles them in the arms of safety, while he that exercises no faith unto repentance is exposed to the whole law of the demands of justice; therefore only unto him that has faith unto repentance is brought about the great and eternal plan of redemption."
Mercy overpowers justice but cannot rob it. Maybe a better way for me to think of it is that mercy overshadows or covers justice. It doesn't overpower justice by disregarding or ignoring it. It can overpower justice only because justice is satisfied. In other words, justice is the foundation upon which mercy is built, it cannot be ignored or destroyed, it can only be covered by mercy.
I think as far as my life goes, I could almost completely focus on being merciful and trust that the Lord will take care of justice. If I am going to make a mistake on one side or the other I would be best off to err on the side of mercy and allow the Lord to deal out justice. I already know that justice will be satisfied in my case either by my own payment or by the Savior's. What remains to be seen is how much mercy I will receive. We know from the scriptures that we can actually influence our own judgement. The Lord taught that we will be judged with that same judgement with which we judge others. When the Lord goes to measure out rewards, He will use the same scale with which I have measured out judgment in my life. If I have been abundantly merciful and generous with my forgiveness and compassion, it is just for Him to use the same measurement in His judgment of me!
In this way, it is fair for Him to use an unfair scale, because it is the scale I have always used in my judgment of my fellow men. This is how we qualify for mercy, by being merciful. Blessed are the merciful for they shall obtain mercy. Christ taught that if we forgive men their trespasses, the Lord will forgive us our trespasses, but if not, we will not receive forgiveness.
This is one reason why it is not only required, but wise of us to forgive all men. It shifts the scales in our behalf and allows The Lord to apply mercy in our judgment. It is only fair that He judge us using our own scale. That is the only just way.
There is a Proverb about how to treat your enemies. It says something about how being kind to your enemies heaps coals upon their heads. While it is not my goal to heap coals on the heads of others, I do desire to have the coals of judgment removed from my head. The Savior also taught us to turn the other cheek, and to go the extra mile with he who compels you to go one, and to give the cloak also to the man that sues you for your coat. This counsel all makes sense when you consider these truths about mercy and judgment. If you hope the Lord will give you better than you deserve, you should give others better than they deserve. The more merciful you are to those who have treated you poorly, the more you shift the scales in your favor.
The Lord has told us that judgment is His. In fact He employeth no servant there. I am so grateful for this truth. I'm so glad to know that I will not be put in a position to be the judge for another, God will take care of that. Knowing this, I can do my very best to judge righteously, erring on the side of mercy, and trust that He will take care of justice. What a huge blessing and relief. I don't want to treat others fairly, I always want to err on the side of treating others much better than I think they deserve.
Teaching moment: Have a child evaluate someone else's performance of a job/chore. Invite them to find everything that has been done wrong or poorly. Invite them to do it again and look for everything that was done well or correctly. You could have them decide how much of a reward/payment ought to be given to this person after each evaluation. Ask them which way they would rather be rewarded by the Lord. Teach them that God judges us based on how we judge others (Matthew 7:2).
Alma 34:15-15, 27-29
Matthew 5:7, 38-48