Complacency seems like a genuine hazard that can feed on itself and accelerate a greater fall into depravity. Is this in fact “the” or “one of the” mechanisms causing the fall of the interlopers and resulting in their twisted nature?

In a sense, this is very, very true. When beings become very self-satisfied and lack higher goals and cease to strive to make themselves better, they are giving up the inner spark that is the yearning of the soul to advance continually in all it does. So this is a trap of the ego, to become complacent thinking one is an exalted being and lofty and has arrived, in the sense of reaching high-level status of capability and accomplishment. It is akin to the phrase "resting on one’s laurels" when there is more to be done and more growth and advancement possible. You can strive mightily but will never reach an endpoint. There will always be more that can be done and accomplished to aid growth and learning. That is how the universe was devised to be, not to be fun for a while but then a dead end where people have topped out and there is little more for them to do that is challenging and entertaining. The opposite is the case when people are not being manipulated, subverted, or distorted in their thinking through a corruption of some kind. That is what has befallen the interlopers—the extraterrestrials have been corrupted by dark spirit beings and this has led to a narrowed perspective and lowered standards. To seek power through subjugation and the suffering of others as a form of amusement and delight are depraved perspectives. No good can come from living that way, and this has been a major setback in progress for all of the extraterrestrials. Living in a love-less world is not just a dead-end, it is a downward spiral, and will end up in a fall from grace, so complacency is a waystation on that path. When one gives up progressing, and in the doing will start to drift from divine alignment, that becomes a slippery slope and soon there may ensue a darkening through lack and inattentiveness and then one becomes complicit in neglect and harmful outcomes to others, and this incurs a karmic penalty and that increases the rate of the downward spiral through the delivering of further challenges, and potentially suffering to the formerly complacent individual, that may catapult them into a state of suffering and misguided behavior that will no longer be complacent because there is discord, but it will not be moving in a positive direction and lead to rewards, it will be a disruption and a discouragement to experience all along the way.