A tyrant zookeeper that occasionally culls the menagerie isn't worse than an omnicidal maniac hellbent on eating everything in sight.
And yet if the tyrant zookeeper continues to torture the animals in the menagerie, every single one of them, for a million years, assaulting and killing the majority if not the totality of every sapient species they encounter, to the point where the very
ascended coalesced SOULS of those species are traumatized after the fact...
At some point, one has done more evil than any
actual omnicidal maniac, in the sense that you can do a lot more harm to someone by keeping them alive to torture them for fifty years than by killing them up front.
The Shiplords' justification is that they have done worse and wilder than any
actual omnicidal maniac, in an attempt to forestall the rise of any
hypothetical, future, successful omnicidal maniac.
...
This is, essentially, the natural product of defining one thing that is so bad that it has infinity weight in the balance of judgment, such that literally anything can be justified in its name.
Which, again, is the product of Shiplord
trauma, not Shiplord
calculation, except in the sense that if you let your trauma do your math for you you can justify a lot of bad things in the name of coping with your trauma.