I'm trying to get people to think about if another species judged us by our own actions.
Judging an entire species by the actions of its ancestors and the bad apples among them is pretty obviously evil. It requires both the "sins of the father shall be visited upon the son" doctrine, and the "guilt by association" doctrine, which are fundamentally unacceptable, punishing innocent people for misdeeds committed by others. If the sentence for that is genocide, so much the worse: a sapient culture trying to exterminate or corral another sapient culture for these reasons is simply evil. Doesn't matter if they have different views on what is acceptable or not; it is simply, fundamentally unacceptable to apply the two aforementioned doctrines to anyone, and to seek genocide for such reasons is that much worse.
This is not related to (purportedly) unique characteristics of human beings. Other species might easily do things (which I will not specify) that humans find squicky or horrible but which are perfectly fine for them. That's not at issue. These beings are known to have three qualities: they are of highly comparable intelligence to humans (definitely not significantly less intelligent), they form a social structure which allows them to act cooperatively as a group rather than each individual member acting on their own, and they understand the idea of communication and interaction with others.
Unless the species is a sci-fi/magical hive mind truly incapable of understanding the concept of individuality (which is a pretty big "unless"), these characteristics require that the species possess certain faculties and knowledge. It is not possible to have a working society sustainable in the long term if its members are incapable of recognizing the commonalities between the members; there may be stratification, even extremes of it, but fundamentally "de-sapientizing" (if you'll pardon the nonce word) their fellows is incompatible with long-term social stability and cohesive action against humanity. A society which punishes guilt by association and which holds descendants indefinitely accountable for the sins of their forebears is one which will very quickly destroy itself, because the only way to absolve the former is social fragmentation, destroying the ability to cooperate on a large scale, and the only way to solve the latter is
suicide. There is a reason humanity figured out real damn quick that an eye for an eye makes the whole world blind. The ethic of reciprocity, whether in the negative form (don't do unto others what you wouldn't want done to you) or positive form (do unto others as you would have done to you), follows pretty close behind as a fundamental guiding principle of dealing with social groupings.
And if this species is somehow able to reap the fruits of a stable, organized society with a consistent group-wide response ("kill/imprison all humans!") and yet
have these principles that are fundamentally antagonistic to the formation of such a society...how? It must be explained. I refuse to accept someone putting it in a black box and asserting that it is simply unknowable to us, that we must simply take the claim at face value and never question it.
Again, this completely ignores any characteristics that might be unique to humans. Maybe you don't need our bizarrely intense bonding instinct which causes us to adore members of other species and instinctively seek to befriend them. Maybe our dangerous curiosity and need to explore beyond the horizon is optional for effective growth as a sapient species. Who knows? But there surely must be things like the above that we can reason about purely from the facts of these aliens being intelligent, able to communicate, and organized by social interaction. (Again, a sci-fi/magical hive mind that lacks that third part would fail to match my analysis, but I am comfortable with that fact. Especially since it would seem to be rather strange if the hive mind is apparently smart enough to reach the stars but too dumb to ever figure out that non-hive life forms are made up of individuals which do not like to be slain.)