Answer:
The reason for participating in the assassination plot against Ceasar that Brutus gives in his soliloquy in Act II is that he does it for the good of Rome since he thinks Ceasar might abuse his power.
Step-by-step explanation:
He justifies his decision by comparing Ceasar to a snake, which you don't want to encounter and face the danger they represent, he expresses his worries about how power after being crowned would change Ceasar and what kind of harm he could cause.