Does Clytemnestra deserve to die? That would seem to be the big question when it comes to dealing with the Queen's character. On the one hand, Clytemnestra did cheat on and then murder her husband with no qualms. In fact, she continues to celebrate this fact on a yearly basis. Clytemnestra also treats her daughter horribly, and hasn't been a mother at all to her only son. On the other hand, the Queen claims that her first husband, Agamemnon, was a horrible guy who sacrificed their daughter, Iphigenia, unnecessarily. (Agamemnon said he sacrificed Iphigenia to the gods; Clytemnestra believes it was for a military cause.)
Whatever stance you take when it comes to evaluating Clytemnestra's crimes, you're going to get stuck applying the same rubric to Electra's crimes at the end of the play. If you think the Queen was justified in killing Agamemnon, then Electra is justified in killing the Queen. In turn, someone else would be justified in killing Electra. "Blood for blood" is a messy system. On the other hand, if you don't believe that Clytemnestra was justified in killing Agamemnon, then you might be forced to conclude that Electra is not justified in murdering the royal couple.