Alma 57:15

therefore it became expedient for us, that we should put an end to their lives, or guard them, sword in hand, down to the land of Zarahemla; and also our provisions were not any more than sufficient for our own people, notwithstanding that which we had taken from the Lamanites.

So we had to choose. We could kill them or guard them with swords all the way to the land of Zarahemla. Also, our food was just enough for our own people. This was even after we took food from the Lamanites.

We had two choices: execute them, or march them to Zarahemla with swords drawn the whole way. And our food supplies were barely enough for our own people, even with what we’d taken from the Lamanites.