Try telling the cops that. In a literal sense "kill" is a verb so someone who has "killed" is a "killer". However, I'd say for some cases they can be described as justifiable homicide and not murder. Catwoman had an unarmed man at her mercy and killed him, as did Grayson. However Grayson then actually tried to do better and hasn't done anything like that since, Catwoman however still continues to break the law, much like Jason Todd. Catwoman didn't try to better herself, she continued to break the law and uses Bruce's feelings toasted her as protection knowing he wouldn't place her in a prison where he thought she would be unsafe. She is still a villain. And thinking about it Blockbuster wouldn't count as killing because it is not a crime to not let someone shoot you, it was more of a failing to do his duty as a hero. Which he then made up for in Infinite Crisis. Whereas Batman killing Sensei would be justifiable homicide as he had killed countless others and intended to kill more but Black Mask could have been incarcerated.