Orochimaru is far more 'evil' then Itachi

Itachi is a murder, Orochimaru murders then experiments on the bodies of his victims, not only that he keeps some of them alive while he does it lately (chapter 199 when the guy asks him to release his clansmen)

basically Orochimaru is evil incarnate in the world of Naruto whilst Itachi is just goal orientated diet evil