Electra is a pressure cooker filled with explosive teenage angst. We can totally see where it comes from in her case. It's got to mess with you when you watch your mom cheat on your dad with his cousin for years while he's at war, then chop him up with an ax when he finally gets home. Electra is totally unforgiving of Clytemnestra for all this, and is hell-bent on getting revenge. Without Electra's urging, Orestes may never have come home to do his mother and Aegisthus in.
It's interesting that Electra doesn't seem to be ticked off at her dad. After all, he did sacrifice Electra's older sister, Iphigenia, to Artemis. (For the full story on this, click here.) It seems like Electra could just as easily have brooded with her mother all those years and then wanted to help Clytemnestra do the deed. (What gives, Electra? Did you not like your sister or something? Did she borrow your clothes without asking?)
It could be that Electra's attitude somewhat ironically represents the attitudes of the male-dominated society she lived in. At the time, the death of a woman just wasn't nearly as important as the death of a man, especially one as powerful as Agamemnon. It could also be that Electra was just a serious daddy's girl. She loved her father so much that she could never forgive his murder no matter what the reason.