Traitor
noun
Someone who backstabs (裏切り) someone is a traitor.
The reading is 裏切り and an exception reading for 者. Just think about how you'd think of a traitor as a "thing" (物) and not a person.
彼は裏切り者なんかじゃないよ。ただ、生まれつきちょっととぼけているだけさ。
He is not a traitor. He’s just naturally a little slow.