First point: Shimei said that David had come to ruin because he was a murderer, and so he had. Shimei told the truth, and all truth is God's truth, whether it is spoken by a madman or not. He was not "innocent" for cursing the Lord's anointed, but David oscillated between repentance and vengeance, and I have no evidence that the latter is more pleasing to the Lord than the former.
Second point: Bearing false witness against a neighbor is still against the Commandments, even when your neighbor is the Pope.
Third point: don't write back to me. I can tolerate malice only in the smallest doses. My instructions are to pray, and I will, Honeysuckle Rose.