In Act II of Julius Caesar, what does Brutus mean when he says, “And therefore think him as a serpent's egg, Which, hatched, would, as his kind, grow mischievous.”?
Antony will grow to become a mischievous but effective ruler.
Cassius should be destroyed before he becomes dangerous.
Caesar might not be dangerous now, but he will in the future.
Decius is serpent-like and therefore he cannot be trusted.

What Brutus mean when he says “And therefore think him as a serpent's egg, Which, hatched, would, as his kind, grow mischievous.”, is that once Caesar becomes great, and reaches his full power, no one will be able to stop him.

Explanation:

According to Brutus, the rulers abuse their power if they have no compassion.

Although he knows that Caesar has never let his emotions outweigh reason, he is afraid that being so young, he will be carried away by power and turn his back on those who supported him. At this point, he does not know how Caesar would act.

That is why he uses the example of the "snake's egg", who once it leaves the shell becomes very dangerous. Therefore, it must be destroyed before that happens.