In Science Fiction, moral ambiguity is a common theme, and the question is often brought up as to whether Humanity, as a whole, is as heroic as we tend to think of ourselves. Humanity can land anywhere on the line of Good and Evil.

However, it seems that there's an easy shorthand for guessing where humans fall on the morality scale, just by seeing what they call themselves!

If Humanity calls themselves "humans" or "earthlings," then they're probably neutral, likely weighted a bit towards the good end.

However, if they call themselves "Terrans," they're probably a lot closer to the nastier end of the spectrum.

There are probably a few reasons for this:

• People aren't used to the phrase "Terran" or "Terra;" it seems alien and unusual, and divides the audience from the Terran characters.
• Its a generally harsh-sounding word; go ahead, say it in a nasty, authoritative voice a few times, it sounds a whole lot more right when you say "Terran!" like some sort of space fascist barking orders at subordinates, rather than saying it softly and gently.
• its Latin (a language well known for its ability to make anything sound foreboding), and sounds similar to "Terror."

Put simply, in science fiction, it seems that "Terran" has become shorthand for "Nasty Human."

Examples:

• Star Trek is the likely Trope Maker here, with the evil, human-run, xenophobic Terran Empire of the Mirror Universe acting as a contrast to the benevolent, multicultural, generally nice-guy United Federation of Planets.
• In Warhammer40000, the Imperium of Man refers to Earth as "Holy Terra." The Imperium is an organization of generally psychotic, xenophobic maniacs willing to incinerate entire planets to eliminate small colonies of Aliens or potential Chaos invasion. The Imperium only comes off as gray because everyone else in 40K is worse, and if they were transposed into any other setting they would be unabashedly and unwaveringly labeled as pure evil.
• The Terran Confederacy of Starcraft, which consists of the descendants of criminals and murderers exiled from Earth and does little to help that reputation, with highlights such as permanently riveting convicted murderers into Power Armor and using them as shock troops or Nuking an entire planet because of a small group of dissidents. Its Successor, the Terran Dominion, is little better.
• Ground Control II's Terran Empire.
• Blakes Seven gives us the Terran Federation.