It's complicated. Obviously, murder isn't forgivable to you, so obviously on some levels and to some people it must not be "forgivable" -- But why draw the line at murder? What about rape? In my eyes, that's worse than murder, as murder can be justified to some degree.
Take these as examples: It's possible the person you murdered did something far worse and would do it again if not stopped. War is mass-murder, yet we justify it every single day, every single time. Should all parties involved be given the death sentence because they are unforgivable? Would that act in itself be murder?
In the end, I could forgive a true friend for anything they did. I have no right to be emotionally involved in anyone else. It's none of my business and I'm no judge or jury.