Brutus and Cassius were both members of the Roman Senate, and they were both disillusioned with Caesar's growing power. They believed that he was becoming too ambitious and that he was a threat to the Republic. They were also concerned that Caesar was planning to make himself king.
In 44 BC, Brutus and Cassius joined a group of other senators in a plot to assassinate Caesar. They lured him to the Senate House and then stabbed him to death. Caesar's death sparked a civil war, which eventually led to the rise of Augustus Caesar and the end of the Roman Republic.
Brutus and Cassius were both killed during the civil war. Brutus committed suicide after his defeat at the Battle of Philippi in 42 BC. Cassius committed suicide before the battle.