Something that's been ruminating, but I'm fairly sure is kosher:
Murder is unforgivable. The sin of killing someone is unforgivable because only the person wronged can forgive, and they cannot. The sin of hurting others through the killing can be forgiven, seen in cases where the parents of the person killed forgive the killer, but that killer can never be wholly forgiven.
However, I hear you shout, where does God's forgiveness come into it? I have no idea. Yet. His forgiveness without human forgiveness must stand true, because otherwise there is a limit to grace. Which there isn't. So murder is unforgivable on human terms, but forgivable on God's.
Maybe I didn't have it down as well as I thought I did.