@DPChaos I wouldn't be so bold as to make sweeping claims about all of humanity. Whether or not people would repent in the "depths of despair" could only be verified by looking at a real apocalypse.
I think it's more productive to look at it from a literary angle. As in, "has Isayama laid enough groundwork for this development to seem plausible?" I would say he could have done more.
As far as I can remember, this is the first time we've seen this fort commander. This means we can't view this development as "out of character" for him as an individual, since we know next to nothing about him. He might well have harbored doubts before this about the morality of Marley's intervention in Paradis. If he knew that the wall king had promised not to use the rumbling as long as Paradis was left undisturbed, then he might see this as the direct consequence of his and his comrades' actions.
However, because we know nothing about him, this repentance feels like it came out of nowhere. Until now, the only Marleyans who had repented were those who saw the Eldians' humanity firsthand; we have not seen a single Marleyan outside of Magath and the warrior corps who didn't blindly hate the Eldians. Had Isayama shown back in the Marley arc who this guy was and what his views on Eldians were, then this would feel more earned.