Manifest Destiny was basically the belief that the US was destined to span coast to coast, regardless of the Native Americans or Mexicans that stood in the way. It’s a very imperial mindset.
Why stop there? Don't forget that the Aztec empire was just that, an empire. They conquered land, sacrificed their enemies to their gods and were an altogether unpleasant bunch which explains why the Spanish conquistadores got help from natives in bringing down the empire. Before the Aztecs came the Chichimeca who conquered their predecessors, the Toltecs. Before the Toltecs came the Otomi, etcetera. Nahuatl, Itza’, Mixtec, Zapotec, Totonac, Otomi, Pame, Purépecha are all colonialist languages - and that covers only the last few thousands of years of history in meso-America.
Indeed it is, something to keep in mind when confronted with accusations of "colonialism" towards specific nations, peoples or cultures. Conquest and colonisation are part of all our histories, no matter our background.
That doesn't make them good, or justifiable. We are all descended from murderers; that doesn't make murder good. It's always happened, it's currently happening, and we need to stop it.
The moment we sit down and start singing Kumbayah some neighbour will take his chances to sweep us up. If ever we were to manage to tame the entire human race into singing Kumbayah we'd be taken over by a fleet of flying saucers, sapient gorillas or just rebooted because the simulation entered a state of deadlock. It isn't just humans either, strife and conquest are a fact of life from the most primitive life forms to homo sapiens and everything in between.
Well, we don't live in the imperial hegemony of the Spanish empire, let alone the Aztec. We live in the imperial hegemony of the United States. The imperial power that is still violating treaties, sterilizing natives, and maintaining "reservations".
It's also kind of gross that westerners need to equate every state with their specific settler-colonial ones. All of those "colonialist languages" co-existed for hundreds of years and were only endangered by the introduction of European settlers. The wholesale liquidation of entire cultures was something uniquely western, and as a consequence the only languages I have left to describe the crime are those belonging to the criminals that did it. In 100 years the Aztecs did not do what the Spanish did in a decade. I'm sure you'll say the Aztecs simply lacked the means, but then you're comparing them to an actual settler-colonial empire.
You say Aztecs were an unpleasant bunch and that's why other tribes helped the Spanish, but as I remember it the conquistadores were welcomed as Gods in Tenochtitlán. Doesn't seem like a very unpleasant welcome, the reward for which was razing the city and killing millions. Seems to me like the other tribes might have helped the Spanish because they knew which way the wind was blowing? Or maybe they also viewed them as gods. Can you ask them how they felt about the Spanish once the Aztecs were gone and the conquistadores enslaved everyone, mutilating people for not mining enough silver?
It’s kind of telling why people need the victims of genocide to have somehow “earned” it, either from being “weaker” or “just as bad”. I choose to not minimize the slaughter of around 100 million indigenous Americans in the "New World". There is no justification for it nor was there precedent.
"As for what we were like before we met you, I no longer care. No periods of time over which my ancestors held sway, no documentation of complex civilizations, is any comfort to me. Even if I really came from people who were living like monkeys in trees, it was better to be that than what happened to me, what I became after I met you."
— Jamaica Kincaid