Alma 42:21 - 42:25
Alma 42: 21 And if there was no law given, if men sinned what could justice do, or mercy either, for they would have no claim upon the creature?
So if there were no laws, punishment could not be given out, that would be unfair. You cannot punish someone for violation of a law if it did not exist. Then again, mercy would be unnecessary as well. You cannot be merciful to someone who violated a law if it did not exist either.
Alma 42: 22 But there is a law given, and a punishment affixed, and a repentance granted; which repentance, mercy claimeth; otherwise, justice claimeth the creature and executeth the law, and the law inflicteth the punishment; if not so, the works of justice would be destroyed, and God would cease to be God.
However, there are laws on the books. Punishments are in place for those who violate such laws. But there is a mechanism in place to get around punishments called repentance. If one repents, mercy can be called in. If one does not wish to repent, then the punishment affixed to the law broken is applied. If no punishment was applied, then justice would be throated and God would no longer be a God.
It seems logical to me that if God would violate His own decrees, then He would not be a God anymore. He may be able to create worlds, etc, but if He violates His own decrees, He loses that status. One of the attributes of Gods is that they keep their word and do not violate laws.
Alma 42: 23 But God ceaseth not to be God, and mercy claimeth the penitent, and mercy cometh because of the atonement; and the atonement bringeth to pass the resurrection of the dead; and the resurrection of the dead bringeth back men into the presence of God; and thus they are restored into his presence, to be judged according to their works, according to the law and justice.
But we know that God is still God so His words are kept. Mercy can have claim on the penitent if they repent of their sins. The atonement takes effect on the penitent. In addition the atonement brings about the resurrection as well. And when a person is resurrected, they are brought to the presence of God and there they will be able to continue in the life style they chose in life. If they were evil they can be restored to the status and if they wanted to be righteous, they would be restored to that life style as well.
Alma 42: 24 For behold, justice exerciseth all his demands, and also mercy claimeth all which is her own; and thus, none but the truly penitent are saved.
So the bottom line is that mercy claims the penitent and justice claims the unpenitent.
Alma 42: 25 What, do ye suppose that mercy can rob justice? I say unto you, Nay; not one whit. If so, God would cease to be God.
Again mercy cannot rob justice. If God went against this edict, He would cease to be God.