IMO, as a believer in Evolution, aliens cannot be that much different than humans. There is only one reality; in order to become an intelligent, space-faring race they had to go through much the same things we did. In particular, our best understanding of evolution today suggests they would be relatively weak compared to other animals on their planet (as we are; most top predators could have us for lunch), and relatively social with each other as well: intelligence lets us take mutual gains by cooperating instead of competing. A group can eat more reliably than an individual, protect itself better, lose fewer offspring, etc.
And groups (e.g. tribes) and sociability are pretty much necessary for developing language, writing, long term history, learning, schools and science that would be needed to become a space-faring race. Aliens would undoubtedly have a society of some sort, likely (like us) bound by emotion, with rules and the "taming" (to use recent terminology) of the members; i.e. we humans are "tamed" by society to behave in a way that preserves our society; just as my dog is tamed to not crap on the floor or "mark" territory in the house, and (my dog at least) stays out of the dog food, won't set foot in the street without my permission, etc.
In your position, as a writer, I would research for particularly "tame" human societies, and kind of base my alien society on that, and have them see the humans as a much more belligerent and violent human society. The Vikings come to mind, they had little regard for non-Vikings and saw them primarily as prey. The only thing they respected in non-Vikings was strength and battle prowess. The alien's history might plausibly include ancient myths of similarly ruthless or psychopathic warriors; which they use as their benchmark in judging humans.
Remember, their civilization is likely to be much more advanced than ours if they are space-faring and we are not. They will likely see us as primitives, as we might see cavemen that hunt and kill every day, are generally unwashed and flea ridden, and resort to violence quickly. Such cavemen, despite having the same mental capacity as us, would not understand a polite modern culture.
So by analogy, we would be unlikely to understand all aspects of the alien culture; except you can probably count on it being very well geared toward high cooperation: getting along, avoiding conflicts or feuds, seeking understanding, etc. Very new age! They've got to live together on presumably long journeys, they'd likely want to keep everybody happy and doing their job without screwing up.
And, to your plot point, they'd want to keep any race that is not geared toward high cooperation with others out of the system. Coming to Earth and seeing we can't even get along with or tolerate each other (racism, sexism, bigotry, wars, etc) is all they need to know to keep us the hell out of the interstellar community.