Agamemnon

Agamemnon was the brother of Menelaus, and husband of Clytemnestra, he was father to Electra and Orestes. His mother and father where King Atreus and Queen Aerope of Mycenae. Legends make him King of Mycenae/ Argos, thought to be differant names for the same area.
 The Trojan war started when Helen was kidnapped, or taken away, by Paris. Helen was married to Agamemnon's brother, Menelaus, and thus Agamemnon commanded all greek soilders to fight troy to win her back.
 When Agamemnon returned from Troy he was murdered. (according to the fullest version of the oldest surviving account, Odyssey Book 11, l.409f.) by Aegisthus, his wife's lover. The story says that the scene of the murder was in the house of Aegisthus, involving ambush and death of Agamemnon's followers. In later accounts, his wife does the killing, or both her and her lover, inside the palace.
 However, If Agamemnon was murdered in his wife's lovers home, this would suggest Agamemnon knew of the cheating, and/or he was friends with Aegisthus. Well, thats what i have just worked out anyway.
It is beleived, that the murder was induced not only by Clytemnestra's love for Aegisthus, but also because he went to war over Helen of Troy. Which, to be honest, is a fair point, she could have beleived he loved her.