Caesar’s murder happened in the immediate aftermath of a civil war and sparked another generation of civil wars, so the actual whys are debated. But in general Romans hated tyrants, and one of Brutus’s ancestors had murdered the last of the Roman kings. If you want to get into the details I recommend this chapter in the Blackwell Companion to Juluis Caesar (bit.ly/bcl-caesar-assassination), which will point you to various interpretations and the ancient source material. (p.s. The line from Shakespeare’s Julius Caesar is “Et tu, Brute?” (“even you, Brutus?”). Plutarch has him say “kai su, o techne?” (“even you, my son?”)