Why would it track that if they are thousands of years more advanced than us, that they would still behave as barbarically to other lifeforms as we do? That's the disconnect that I always have issue with, whenever someone suggests the "aliens will just kill us" theory. Especially since they so often discredit the "aliens won't be assholes" theory, immediately before stating their own stance is more accurate. Despite neither having anything other than pure speculation. But it often goes "They will be SO advanced beyond us!! " Ok....probably yes. So why wouldn't they also advance beyond us on how to behave to other living beings?
And regarding your points about animal testing, a lot of humans are very much against animal testing, and using animals as food sources, BECAUSE of the ethical stance. So it's not even a universal stance for us cavemen. That's the point that I take issue with. Your stance assumes nothing but technological advancement for thousands of years, while excluding any level of social/ethical advancement to go along with it. Which just doesn't seem to track very well, given our one sample species of ourselves. I mean the ethical stances of your average human society just 200 years ago, are vastly different to our view of the world today. And then to compare it to 2000 years ago, it's not even the same concept of existence. Years of fighting for rights, learning new aspects of how the world works, have had radical changes on our societal/ethical outlook. To think the aliens wouldn't have the capacity for that as well, just seems really narrow-sighted to me.
Now, I fully acknowledge, that I could be wrong, and they could be the equivalent of intergalactic, FTL tech wielding Mongolian Horde, and just sweep across the galaxy, but I don't think that any species, with the level of advancement needed to accomplish FTL and similar, would think that to be a good way to exist.