It's true that Spock might consider what did as murder no competent court would convict him. In the eyes of the law he did nothing wrong. Seeing as he had just emerged from Plak Tow and thought he had killed Kirk, his opinions would be suspect at best. His emotional reaction to seeing Kirk alive also shows that he was not in his usual mental state. He might have presented himself at a Starbase but, assuming the JAG office is doing their job, no charges would be filed. He wouldn't offer a defence because none would be necessary. He may feel guilt for killing Kirk but he had no reason to consider it murder.