Was the United States justified in using the concept of Manifest Destiny to defend their westward expansion?

No, they were not. The Manifest Destiny said that the land was rightfully America's because it was given to them by God, and that the Native Americans did not know "how to use it properly". Native Americans did know how to use it properly, they just had no concept of land ownership since they thought that the land was for sharing and it belonged to the Earth and not humans to manipulate. White Americans and the government used this belief to their advantage and said that according to the Native Americans, since no one owns it, they own it now so it is rightfully theirs. They used the concept of the Manifest Destiny as a justification, so when they wanted territory and needed an excuse or justification for a push into territory that they did not control, they used in Manifest Destiny.