Kharn was in some respects "the voice of reason" for his legion in the period leading up to the Siege of Terra. He was equerry to his primarch, he was the guy who finally broke Angron out of his "kill all own his legionaries" rut that he fell into after the Emperor rescued him from the gladiator planet, and so forth.
In Betrayer there seems to be a strong trend of him being less crazy than the average berserker.