There are different ways to look at Planet. One is to view it as a pristine place, akin to a temple, to be left alone to the gods, or preserved into some distant future for purposes yet unknown. A sacred place which hold many secrets, which shall remain secret for the good of the galaxy.
You can view it as a big, fat, juicy battery filled with enough raw power to catapult yourself and your closest millions of friends directly into manifest godhood, just waiting for you to come around to push the metaphysical button. After that, the galaxy is your playground, to conquer at your leisure.
To say that the Usurpers are dedicated towards achieving transcendence would be a monumental understatement. It is the reason they arrived on Planet in the first place, and after the crash landing it is just about the only way off of this rock. The stakes are godhood or death; everything else is either a step towards the former, or a step away from the latter. To quote a famous rock group: nothing else matters.
This fanatical dedication to transcendence defines everything about the Usurper faction, as you might imagine. This includes its relationship to humanity. The fact that they in many ways share a common predicament with the humans – humanity, even though it does not know it yet, is also caught between transcendence and death – only serves to underscore just why peace is an impossibility. The very similarity makes it so: if humans are allowed to live, they will inevitably progress towards their own transcendence, potentially beating the Usurpers to it. The relative backwardness of humanity and their ignorance of what is truly at stake, only makes it more imperative to get a head start while the getting is good. In a race where there can only be one winner, fairness and even playing fields are to be avoided at all costs.
In mechanical terms, this translates into the Usurpers (and the Caretakers, too) being manifestly overpowered from the word go. A player who chooses the Usurper faction will have no difficulty steamrolling their opponents (very much including the Caretakers), and given the nature of the faction advantage, it only grows larger as the game progresses. While this is understandable from a narrative perspective – high-powered scientifically advanced aliens rebuilding their material foundation after a violent crash landing – it does highlight how perspectives on game balance (however asymmetrical) has changed over the decades. There are no “humans are weaker in the early game but stronger later on” considerations; the aliens are just stronger, full stop.
This does make for some interesting gameplay considerations, however. Playing with the seven expansion factions means being permanently at war with two of them (the aliens), and having diplomatic options with the other five. Or, if playing as either of the aliens, being permanently at war with the other alien faction, and ever so gradually finding yourself at war with everyone else. Whether this was intended, or an artifact of the Civ 2 era diplomacy system, where you could be at peace for years and years until you clicked on the communication button, thus reminding the other faction that a) you exist, b) that they hate you and c) this is a sufficient reason for an immediate declaration of war, – is an open question. Either way, playing without the aliens makes for somewhat more peaceful runs.
Speaking of unfair advantages, the aliens have an additional victory condition: they can construct subspace generators. Upon building six of these, a portal is opened to the home world, wherefrom untold legions can be summoned. While the game ends after their construction, it is heavily implied that the sheer quantity of troops, weapons and advanced tech brought forth through the gate overruns everyone else on Planet combined, and that resistance after that point is symbolic and utterly futile.
The question, then, is what the Usurpers intend to do once they have achieved their goal, either through local or external means. While the question, like for the other factions, largely remains unanswered, we do glean two things from the leader quotes of the respective alien factions. From the Caretakers, we learn that something like this has happened before, with terrible consequences. And from the Usurpers, we learn that despite all that, it is still worth it:
Risks of Flowering: considerable. But rewards of godhood: who can measure?
– Usurper Judaa Marr, “Courage : To Question”
Protagoras once said that man is the measure of all things. The Usurpers, utterly alien as they are, beg to differ.