This is tricky, in your example the correct sentence would be "He has been killed". Typically, "has" is used to refer to something in the present, "has" implies more urgency like the killing was very recent or just happened. "He WAS killed" is very matter of fact, only used when it has happened much before, usually when using "was" a date and time can also be specified. You will here "has been killed" in a lot of TV Dramas because it is more suspenseful, it implies that the killings are happening right now. Hope this helped :)

The most important thing to say is "has been being" doesn't make any sense. If you mean "has been killed" then this would mean simply "he has been killed". But "he was being killed" Is not complete on its own and needs more to this sentence or more context to make sense. Since you must be talking about something else happening at the same time as him being killed.

瞅你是同胞，那就母语给你解释吧。英语就没有HAS BEEN BEING DONE 这个说法，如果你在某书上瞅见了，恭喜你，碰上盗版书了，万一真是正版的，那你可以去索赔了，因为垃圾污染了你的智慧大脑！was being done 是表示被动语态的过去进行时，比如你写回忆录提到上海那个楼歪歪就可以说 the building which was being built in 2005 fell down suddenly .晓得伐？