In the killing of Agamemnon who is the lion's cub that turns into a killing beast as an adult?
The lion cub would have to be Orestes, the son of Agamemnon and Clytemnestra. When he returned to Mycenae from the Trojan Wars, Agamemnon's wife was not very happy to see him. One reason why was that before setting sail for Troy, he had sacrficied their daughter Iphegenia in order to guarantee good winds on the voyage. Another reason was that while he was away, she had taken a lover, a man named Aegisthus. Together, they had ruled Mycenae and did not want to give power back to Agamemnon. So together they murdered him.
Orestes was very young when his father went to war, and he was now a young man. By rights, he should be the new king after his father's murder, but he had a powerful mother and her lover to contend with. Orestes fled Mycenae, but he swore to get revenge. His sister Electra helped him to return to his rightful home and brutally murder his mother and her lover. So the lion's cub grew into a killing lion.
Orestes did not live a happy life after he got his revenge, though. For the rest of his life, he was tormented by the furies.