How we cite our quotes:
(Orestes): "You killed the man you ought not; so you must suffer the thing you should not." (930)
These are the last words Orestes speaks to his mother Clytemnestra before driving her into the palace to be killed. (Whether he says any further words to her once inside, of course, we cannot know: the final killing happens offstage.) They also boil down to its essence the BIG PROBLEM of the nature of Revenge versus Justice that is central to the Oresteia, and that we've been talking about so much in this module and in those on the other plays in the series.
On one level, it looks like Orestes is talking about justice, in the sense of getting what you deserve: "You did X, so you must suffer X." But if you take it down a notch from the level of algebra, plugging values into the X's, you end up with a paradox: "You did what you shouldn't do, so you must suffer what you shouldn't suffer." But if Clytemnestra is going to suffer what she shouldn't suffer, how can that be just? These are the sorts of questions the audience (including us readers) is likely to ask in hearing these lines.
Of course, in his own mind, Orestes probably means something much more basic: "You did what you shouldn't do (i.e., a wife shouldn't kill her own husband), so you must suffer what you shouldn't suffer" (i.e., a mother shouldn't be killed by her own son)." Does interpreting these lines in the more basic way that Orestes intends them change anything? If so, what?