As a Christian, here is my belief about forgiveness:
God can and will forgive any sin of a truly repentent person. But as others have pointed out already, that's only from one perspective. The sinner must forgive himself. And those he has sinned against may or may not forgive him.
Setting aside the issue of whether the sinner forgives themselves, your first comment about "can and will forgive any sin", does that truly extend to anything? Genocide, murder, worse...and there is worse, these sins are forgivable if the sinner is repentant? A good direct example here would be the Son of Sam, he killed several people in NYC because a barking dog told him to, in prison he had a supposed crises of faith and is now an ordained minister, you can buy his sermons on the internet.
Also, forgiveness by others, or by God, doesn't wipe away the real world ramifications of the sin. Ted Bundy may have been contrite, and may have even been forgiven by God (no one knows except God on that one), but he still destroyed lives and permanently scarred families, and he still was executed, even though in his final interview he seemed truly horrified by his own actions, and truly repentent.
I've read that interview, and many of the ones that came before. Most interpretations of Bundy's remorse was that he was afraid of dying, not that he had any feelings one way or the other over his actions. Even his own defense team called him the truest sociopath they'd ever witnessed. So I won't use him as an example, but what about more complex situations.
Nazi Youth for instance, or the "Wild Boyz" of Rawanda, Congo, Sudan - these are children who at ages as young as 6 are forced to murder, often their own families, drugged, and made to do any number of atrocities, made into children murder soldiers, using drugs, rape, multilation as weapons of war. Whats happens to those children later? Some of the so called Wild Boyz from the Rawandan Genocide in the mid-90's are now in their mid-late twenties, some have moved to other conflicts zones and are still doing what they learned to do as children, but some have fled to other countries and live with what they did. How does someone like this find peace? I guess it's really three questions, first a person is directly responsible for dozens, maybe hundreds of rapes, mutilations and murders. Can God forgive them if they are truly sorry and willing to do anything to repent? Second, should society forgive them, should they be allowed to be a part of society outside prison ever? Third, going to self-forgiveness, what could someone like this ever do to make up for their actions?
S3v3n