Clytemnestra comes off a lot differently depending on who's talking about her. If you ask ancient writers in the anti-Clytmnestra camp, she's an adulterous queen who sleeps with her husband's cousin while he's off at war and murders the dude when he gets home. Her son, Orestes, who later kills her for Agamemnon's murder, would definitely agree with this point of view. There's no getting around the fact she's totally guilty of regicide (killing a king) and mariticide (killing a spouse). If she committed these crimes today, she would probably be locked up for life and might even face the death penalty, depending on where she lived.
Just to play devil's advocate, though, Clytemnestra does have a lot of reasons to be majorly ticked off at Agamemnon. For one, the only reason Clytemnestra is married to the guy is because he killed her former husband and claimed her as a prize. (Oooh, romantic.) On top of that, he honors that marriage by sacrificing their daughter Iphigenia to make up for his own crime against Artemis. Not that we're condoning murder of anybody for any reason, but you can kind of see where Clytemnestra was coming from. Her husband was about as bad as a husband can be. While it's easy to pigeonhole Clytemnestra as an evil, murderous adulterous, you can also see her as a woman finally asserting power over a man who treated her like total crap.