Humans aren't all evil, but by comparison the worse villains in intergalactic history were all human, despite the rest of the race being accepted and civil.
Human violence is an uncontrollable instinct.
Violence and aggression are essential for human survival, evolution and innovation. The aliens didn't go down this route.
It's actually averted, but for other reasons the humans that would try their best to get along with everybody are forced to stay away from the borders of the human empires, effectively shutting off any humans who only and peace.
Subverted: Humans are gruff and harsh, and their cities are a grim sight, but in the end it turns out to be that their moral standards are actually higher than those of the alien species, who live in a Crapsaccharine World.
Double Subverted: ...But then it turns out that any redeeming qualities that humanity seemed to have don't really exist, and humans are really as bad as they seemed to be at the beginning of the story.
The level of good and evil varies greatly by species. While aliens may consider lying on a daily basis as unforgivably deceitful and capitalism as disgustingly greedy. Meanwhile their concept of honor is viewed as utterly disproportionate and their insistence on food being still living barbaric. Some of both species fit the other system of morality better.
Averted: Humans are neither particularly good nor particularly evil compared to aliens.
Enforced: "Hey, those fucking green bastards can't talk to us like that!"
Invoked: Humans have been created as a Servant Race to a more intelligent, manipulating race. The "master" race makes sure that no genuinely nice human lives a happy life, no evil deeds go unrewarded, and puts neural implants to the brains of every single human to program them into heartless psychopaths.
Defied: The human ambassador gives alien children lollipops and shakes the hands of every alien he meets. That's after making sure the lollipops won't poison and his hands don't have a disease that would be very fatal to the aliens.
Discussed: "Man, we're such assholes, aren't we?"
Conversed: "Unbelievable! How dare those humans act so rude to us!"
The aliens, realizing their hypocrisy, right their ways. However, the humans begin a vicious cycle by using the aliens as a scapegoat for violence in return.
Alternately, this public relations representative has a lot more influence than the aliens realized, and though not all of humanity is JerkAssly, humanity still rises up under their corrupt leader's flag during an ensuing species war.
Played For Laughs: The human ambassador is a pompous asshole who whizzes on the feet of the alien king, kisses the alien queen, kicks their pet, and gives the chancellor a wedgie.
Played For Drama: Humanity, upon first meeting another sentient species is immediately branded as a race of bastards, whether it be justified or not. Humanity must continuously prove this to be true in order to survive, leading to an unending war between the two or more forces.