After all the other conspirators had stabbed Caesar, Brutus was the last one that hadn’t participated. Before he gave the final blow, Caesar said “Et tu, Brute?” which translates from Latin to English as “You too, Brutus?”. Reading those lines, it’s very obvious that Caesar felt betrayed and let down that Brutus was a conspirator as well. It’s not noble to kill someone, let alone a friend.
Brutus’s reasoning