Basically by definition, revenge makes you live in the past and try to make the past present. If you're out for revenge, that means you're preoccupied by a wrong that happened a while ago; thus, Orestes and Electra are both preoccupied with the murder of their father Agamemnon by Clytemnestra. By trying to get payback, you're basically making the past present, as Orestes intends to do by doing the same thing back to Clytemnestra that she did to Agamemnon. There are a number of obvious problems with this idea. Most basic is: What gets solved? If you keep living in the past and making the past present, how does one ever escape from the cycle of revenge? This is one of the big questions that the Oresteia as a whole wants us to ask.
The original audience of Libation Bearers would have seen it performed right after Agamemnon. This insight into her and Agamemnon's past would have made them more sympathetic to Clytemnestra than they would have been otherwise.
Libation Bearers shows that knowledge and ignorance about the past are equally dangerous.