Someone claims that God is omnipotently forgiving. One of the evidence of this forgiveness is that a murderer like David was chosen to be King of Israel. No, I do not believe that. I think "God" is just an excuse of humans' behaviors - when we need forgiveness, he "forgives" (otherwise how could we explain David was a King?); when we need to be judgmental, he judges (such as homosexuals, heretics, witches, scientists)!
No, I don't believe a murderer is forgivable, unless this murderer did something to compensate his/her crime. Otherwise, how dare are we have legal system to punish those who even God forgive?