The title of the question alone ("Pirates of Earth" at the moment, and I hope it doesn't change) is pretty cool. Sounds like an Edmond Hamilton pulp SF adventure.
Anyway... @santiago, if you are looking for a reason for Terrans to be notably, saliently piratical compared to other starfaring races, you have some heavy lifting to do.
What makes a pirate?
In Earth's nautical history, piracy tended to be a short-lived social phenomenon, one that comes and goes. Typical drivers of oceangoing piracy include:
- Lots of sailors without employment (especially after a war, when governments hastily reduce their expensive navies)
- Valuable cargoes being shipped by sea
- Reduced naval patrols in the areas of piratical activity
These factors tend to be self-correcting, or at any rate short-lived. However, in their heyday, they create surges of piratical activity that seem to erupt, and remain intractable, for a while until the conditions do in fact change.
(See The Golden Age of Piracy.)
However, there's another kind of piratical scenario that may be more useful for your purposes, because it's got more staying power: Asymmetrical warfare.
A handy example is the modern-day situation of Somali piracy, in which the pirates began as fishermen whose livelihoods were reportedly wrecked by toxic waste dumping and illegal overfishing in their traditional fishing grounds. According to the article, the fishermen-turned-pirates have, over the last few years, mostly become criminal gangs rather than resistance fighters, and enforcement efforts have severely reduced the Somali pirates' activities.
However, this is a somewhat different situation than the Golden Age piracy.
Rather than groups of displaced skilled predators (overwhelmingly men) living mainly in isolated enclaves (Tortuga, Nassau etc), the Somali pirates are a self-sustaining population of indigenous people.
Somali piracy has been suppressed by focused, strenuous efforts on the part of the still-powerful Western industrial governments, who are not divided by war or other competition.
So, for your Terrans-as-pirates scenario, just put the Terrans (or perhaps the inhabitants of the entire Terran Solar System) into a position in which they feel forced to adopt asymmetric warfare: using small numbers of FTL ships to do commerce raiding rather than attempting to face more powerful races' starfleets.
What you need to sustain, in your story, would be the following circumstances:
The menace of the antagonistic alien species is unrelenting (they do not stop being malefic.)
The aliens are nonetheless not powerful enough to stomp humanity into submission effectively: the pirates must retain at least some freedom of action.
As long as you can keep these issues reasonably stable, you have in fact created an opportunity for a pirate-friendly system civilization.