Here Brutus is deliberating about killing Caesar. He is talking about killing Caesar, even though as he speaks there is no real solid proof that Caesar is going to become a corrupt tyrant (which is their fear). However, he is trying to rationalize becoming part of the conspiracy. He is comparing killing Caesar before he can become a tyrant and do considerable harm to killing a serpent\'s egg before it can hatch and bite someone and cause them harm.
If you examine Brutus closely, you will see him constantly trying to convince himself that killing Caesar - without any real proof that he will become evil - is the right thing to do. Also note when Brutus states that if it should make Rome a better place, he would gladly kill himself. Brutus, unlike Cassius, is only interested in the good of Rome. Cassius, on the other hand, is motivated by power and greed. In an ultimate irony, Cassius is more likely to become an evil tyrant than Caesar.