Anything can be forgivable, it all depends on the circumstances of the events that took place. Even an extremely evil act can be forgiven, if that person feel remorse for their actions and forgive themselves for what they have done. Even if they do not meet these conditions, you can still forgive them to make yourself feel better.
It is possible to forgive atrocities if the people who did it genuinely seek forgiveness. It is very difficult to not only forgive but ask for it. It calls for admittance to guilt on one side and an admission that another side cannot undo the harm of another against them.
Sure, it's possible to forgive atrocities. Individuals can forgive whoever and whatever they want for any reason. But just because you say you're sorry and I accept that does not mean that I will now like you. It doesn't mean I'll now associate with you. It just means that I forgive you. You're still a monster.
It is not possible to forgive atrocities. This is why they are considered atrocities, because they are just so bad that they can never be forgiven. Would anyone ever forgive Hitler for the Holocaust? Of course not, and that further proves the point that atrocities can not ever be forgiven.
An atrocity is defined as an extremely wicked or cruel act, typically one involving physical violence or injury. I do not believe it is possible to forgive these types of act because they are deserving of justice and punishment. Forgiveness may come after but it should not be directly expected.