CriticaLink | Aristotle: Poetics | Guide to Book XIV
The perpetrator of the destructive action can act consciously, aware of the identities of those he or she is harming. The murder of Agamemnon by Clytemnestra is an example of this kind of action.
The perpetrator can act without awareness of the close ties between him- or herself and the victims, as in the case of Oedipus.
The hero can be about to commit a conscious, destructive act against known persons but then fail to follow through with the action.
Finally, a person can be about to act against those close to him in ignorance, and then be rescued from the deed by learning of their identities in time.
Aristotle dismisses the third kind of action as being completely outside the domain of the tragic, for there is no disaster. The first kind is much better, because at least something happens, but even better is the second, in which the disaster is compounded by a revelation of the close ties between perpetrator and victim. Somewhat surprisingly, Aristotle views the fourth kind as the best, when a destructive action is avoided just in time.
Aristotle concludes by observing that it is not unsual that poets should look to certain unlucky families, such as those of Agamemnon and Oedipus, for material for tragedies.