content: Treason against the crown, punishable by death. The king and queen are not the crown, the crown is it's own entity that endows regency on those of the royal line, if he were to kill her then he will have committed treason against the crown and be sentenced to death, even if he were to somehow secure the king title (He wouldn't as he is a consort, not next in line) he would still have committed treason against the crown he now wears and would be executed even if he wanted to wear it while they carried out the verdict. He would be wearing his accuser while the axe fell on his neck, he can't absolve the sentence because he doesn't have the power to supersede the crown, in the laws only god can supersede the crown. Weird ancient laws aside if he killed her he would have no connection to the throne, he would just be a widow crown consort and have no real power outside of family ties. The crown would pass to one of his children and he's be thought of as a regicide and be beheaded as one, which if they wanted to do so could be carried out by the new king himself as per law.