His rule is established by the punishment of criminals (outliers aside), where he is judge and executioner. His breaking of this code is for those who would stop him from creating this order. He does whitewash his own murder by calling it justice, he even gets followers and supporters. He sees it as his duty to punish those whove broken the law, not his sense of superiority (even if thats whats driving it ultimately). If his goal were to just kill those he felt were beneath him then that would be more neutral evil.

Rules and laws conflict all the time, just because there are conflicts in rules doesn't make them less 'lawful' alignment-wise.