He was killed = passive form - very general way of saying indirectly that a person was murdered He was killed of = passive form again but the difference here is the verb choice. "to kill off" normally means that someone has been removed from a tv show/film or whatever through being murdered. Less commonly it can mean what rebecca said, in the sense that someone has been killed off (like the verb "to pick off") meaning they have been murdered because of something. Often this is applied to organised crime. As you can see this is a lot more specific that above and when in doubt just say "He was killed"

"he was killed" is a general statement that can be used when expressing that a person (male) has been murdered or put in a state of death. "he was killed off" has the same meaning but used in a story telling setting. For example, if I was discussing a show with a friend and I wanted to relay that one of the characters died, I could say "yea, Stan was killed off." Hope this helps.

to be "killed off" generally means a character in a novel or a play. That character was killed to end his or her part in the story. As someone above noticed, you could also use "kill off" or "killed off" in the context of the mafia. But if you think about it, it means the a similar thing: (killed + end of a relationship) or (killed + end of involvement with something)

He was killed: He died for some reason. It could be an accident (earthquake, traffic, etc.) or violence He was killed off: Someone murdered him. A person killed another person. Often used for mafia type descriptions/organized violence.