Brutus: and therefore think him as a serpent’s egg which, hatched, would as his kind grow mischievous, and kill him in the shell. based on the bolded simile, what is brutus’ reason for assassinating caesar?

Just an assumption but I study theatre and have read this play.

Brutus' reasoning for assassinating Caesar, as stated in the metaphor, is backed by his belief that Caesar's rise to power could lead to deadly consequences. Like a serpent's unhatched egg, Caesar seems benevolent at the time, but he could very well end up taking advantage of his power and using it against his kingdom and harming innocent people.

Hope that helps!