Through many AUs I've been through, yours are the one I like the most, because it's a more realistic one.

I can see that the "cruelty" of other Collectors are not like that they are evil.

For a being that is older than universe, they are like the Gardeners of Stars, as a gardener you have to cut it out what is dead, you have to treat the soil, prepare it for a new tree, now with planets it's kind the same, they visit, they watch, and so they decide if some species is worth to keep and if they are not worth for keeping.

And so, this process of leaving a juvenile collector in a planet till the dominant species dies is kinda a hardening process, in order for them to grow mature and to not let feelings intervene in the decision process, like being a doctor, you must put aside the fear of hurting the patient in order to heal the patient.

But in TOH things got different, they found a species that represented danger to themselves, so they used another species to kill the titans and the little collector paid the piper.

Im very glad you enjoy the AU:D

The concept is rooted in the idea that generally, people or characters don't choose evil simply for the sake of being evil. But nobody is omnicent, they react to whats happening, trying to figure out what might be "best" as they go without really a way to know for sure if its a right call. Having power to destroy a planet with swipe of finger rises the stakes for literally everyone. When the Collector was releashed during King's Tide the game changed. If Belos had managed to control them - nobody would have been able to challenge him. Even Odalia tried to suggest totally reshaping the isles. Seeing anyone as mostly/ only dangerous power sources creates power imbalance, something that can evolve into very shaky and actually dangerous relation when the other side realises they were never really considered a equal person and having the ability to revange. There is a lot of implications and possibilities when someone possesses such power with no oversight and unlimited time but also is a person that doesnt want to be alone:D

If involvement with mortals ends in some kind of complications the collectors will be around to see the consequences, even if they don't directly experience them so sort of desensitization toward the very life they are trying to preserve is bound to happen. "They live for so short and can cause so much change in their own system, its best to control the situation" type of mindset. Also thinking of ecosystem like gardens that need work on makes it easier to deal with, especially since with the scale of galaxy they cant just spend unlimited amout of time in one place full of creatures that do not want to be preserved. Their actions come from a place of care but there is inherent cruelty in their concern

Sooo yeah, its the perspective that might develop in that kind of situation and might end up with leaving one of their own alone for eons. But who knows, this AU is a lot of theories in a trenchcoat and i dont want to defend their actions. Killing all titans? yeah thats bad. It's more about theorizing why anyone would consider that a reasonable option while also not being evil just cuz