Blah blah humans are terrible let's make sociological commentary blah blah no thanks. It's been done, and poorly.
I like Wh1sper's idea, personally. Short version at the very bottom:
Maybe they're actually us, from the future. Our genome experiments have allowed us to explore the depths of the universe, to brave the rigors of space travel--rigors involving forces we can only begin to comprehend right now.
Well, some of us. In fact a rather small portion of our trillions upon trillions of humans throughout the galaxies. The side-effect is that the treatments to these rigors make those explorers less and less human each time. Meanwhile the majority of humanity lives relatively unchanged, their lives enhanced by genome therapy.
Except that at some point, an error occurs. A problem of some sort; something was overlooked in the intervening centuries, maybe it was a step discarded as "irrelevant" and yet was crucial to keeping a particular balance in the species. The misshapen human space explorers return to their planets to find worlds of dead bodies. The explorers have a rather limited fertility, and they were explorers--not genetic therapists. So this tiny percentage of future-humanity gets together and develops a last-ditch plan to save humanity: They need to go back in time and stop us from developing this terrible genome therapy.
Unfortunately the journey through a black hole's event horizon
or some other such psuedo-scientific explanation of how they got back in time only exacerbates their already daunting problem. They arrive around 1800, and begin reconnaisance and experimentation. Perhaps their medics and science-officers can find something in "pure humans" to revert their problems? Sadly, language and communication has changed so much that these "aliens" don't really even think on the same level as us. They're also hideous to us, and not likely to be believed. Plus, they know that if they just stop in to say "Hi, we're you, and you need to not experiment in genome therapy," we would just reply with "Hi, please stay still while we experiment on you for our genome therapy projects."
Eventually these explorers/future humans begin dying from their time journey. They invade because it's their last chance: Force a paradox that at least allows humanity to survive.
So that's it in a nutshell: They're future humans, come back to keep us from doing something that seems like a fantastic idea at the time.