I think most anything can be forgiven provided a lesson be derived from it, by all parties. If not, then it's a waste, and fruitless.
A valuable lesson learned heals a whole host of injury. It atones. It makes up for it. It balances the scales. It works miracles, in my experience. It buys time; it ransoms.
I think there's probably some things that cannot be forgiven, however. Genetic meddlings, for example. Overreachings that will plunge us back into darkness again. Certainly there have been previous cycles of "group karma" (or perhaps "world karma"). And certainly its reasonable to consider the possibility, if not the likelihood, that the humanity of each succesive cycle did achieve to genetic manipulations, did achieve to self-replicating nanotech, did break through to the very heart of matter at it's simplest constitution. From that, it's just a short hop to a consideration that it was man's overreachings, not meteors, not a polar shifts, not atmospherics -- not that these couldn't take place -- the lesson is that we can't help ourselves.
Witness the pantheon of mythical half-man half-beasts. Room to consider the naghual as generational throwbacks -- a tribal fear of unspeakable horror carried in their genetic memory. Room to to consider any birth anomaly (the devil's spawn in bygone days) as testimony to genetic diddlings. How many are hushed up in hospital? They arise in the genetic pool as haunting reminders of what we don't know we've forgotten.
. . . and neither will we remember in time this time either.
. . . and so the question for me becomes: What happens when the group karma reaches an exponential curvature -- or has it already?