Frostmourne does not give him an out. He was in full control as shown in the game, in his own book, in comic books based around him. All it did was give him a sense of justification in what he was doing but he was still Arthas, he was still in full control, everything he did he did as Arthas. Just like everything Vader did in Star Wars he was in full control and was still the same person that was picked up on Tattooine as a child by Qui Gon.