Final answer:
Brutus wants to avoid Cassius being involved in dishonorable activities to maintain the moral and noble justification for assassinating Caesar in Shakespeare's play.
Step-by-step explanation:
Brutus doesn't want Cassius involved in "shady dealings" because it will taint the nobility of the assassination. In Shakespeare's play, the conspiracy against Caesar is portrayed as a noble and patriotic act, done for the good of the Republic. If Cassius were to engage in dishonorable activities, it would undermine the moral high ground that the conspirators, including Brutus, are attempting to maintain. Their goal is to remove a tyrant, not to become corrupt themselves, which is critical to justifying their actions to themselves and the public.