Opposite Promises

When the Anti-Nephi-Lehies took that name upon themselves, they also took an oath that they wouldn't fight any more, even to defend themselves. When their enemies attacked them, they allowed themselves to be killed rather than fighting back. Later, they relied on their new allies, the Nephites, to protect them. But when they saw that the Nephites were having a hard time fighting their enemies, they wanted to help, but they kept their oath and did not personally join the fight.

However, their sons had not made any such oath when they converted to Christianity, and when the Nephites' need became desperate, they made an oath that went completely against the oath their parents had made.

And they entered into a covenant to fight for the liberty of the Nephites, yea, to protect the land unto the laying down of their lives; yea, even they covenanted that they never would give up their liberty, but they would fight in all cases to protect the Nephites and themselves from bondage.
Alma 53:17

The interesting thing is that both of these were good oaths. It was good of the Anti-Nephi-Lehies to commit themselves to peace, and it was good of their sons to commit to fight to protect that peace. While it would have been a contradiction for any one person to have made both oaths, each of those oaths were good ones. They were just good for different people and at different times.

Most moral laws are absolute, but the most righteous course of action can depend on the situation. God once said, to one people, "Thou Shalt Not Kill," but He also told others that it was necessary for them to kill some specific and especially wicked people. Fighting is usually wrong, but fighting to defend yourself and the innocent is usually morally justified and morally right.

So when the Anti-Nephi-Lehies promised not to fight any more, not even to defend themselves, that was a good promise for them to make, and when their sons promised that they would fight to defend themselves and the Nephites, that was a good promise for them to make. There may be some actions that are always right or wrong, no matter what, but with other actions, what's right or wrong for one person might not also be right or wrong for another.