Brutus is already worried that Caesar will abuse his power as king, then he reads the anonymous letters and decides Caesar must die.
Explanation:
As Caesar shows an unstoppable thirst for conquering the world (as his inspiration was Alexander the Great), Brutus was already worried that Caesar would change the political realm to be one of dictatorship and privileged on monarchy, ignoring the interests of the people. Brutus's worry is further supported when he received the anonymous letters on his windowside.
