Brutus is the best friend of Julius Caesar. However, he took part in the conspiracy of killing Julius Caesar.
He loves Caesar but he does not want Caesar to be the King. He was torn between his loyalty to his friend and his loyalty to the Roman Republic.
It is very ironic because a friend should always have your back, he should be the one to protect your back. Instead, Brutus, Julius' best friend, was the one who stabbed him in the back literally.