No he's not since an antihero is someone who's the main character but lacks the qualities typically associated with a hero. Think Batman in this regard. Kenshin embodies the hero qualities fully. As is evident by his desire never to kill, represented by his sakabato (reverse blade), and his willingness to do what ever it took to protect the innocent. Even if you delve into his past you still wouldn't find he fits the definition of antihero. In Samurai X, he's more of a tragic hero since he still has such things as honor. He's just under the assumption that killing is the only way to bring in the new era. As for saying he's not a murderer in the actual series, that's false. While they don't use that word, they do say he had killed in the war.