In Julius Caesar, what proof is there that Cassius wants what's best for Rome?

In grappling with Julius Caesar, readers often focus on Brutus' noble intentions. However, there is some evidence that Cassius also wants what's best for Rome, and so it's worth analyzing the character of Brutus' co-conspirator as well. 

The following quote from Cassius in Act 1, Scene 2 is a good place to begin when examining Cassius' altruistic intentions:

Rome, thou has lost the breed of noble bloods!

When went there by an age, since the great

flood,

But it was fam'd with more than with one man?

When could they say, till now, that talk'd of 

Rome,

That her wide walls encompassed but one man?

Now is it Rome indeed, and room enough,

When there is in it but one only man. (151-57)

In this quote, Cassius bemoans the current state of Rome, saying it has lost its nobility by idolizing one man (Caesar) and setting him up above the masses. Furthermore, the passage suggests that Cassius values Rome's noble history of political freedom, and does not relish the thought of a dictatorial rule. Taken in this context, one could surmise that Cassius, like Brutus, wants what's best for Rome.

However, it's important to also recognize that Cassius is, unlike Brutus, prone to jealousy. It's no coincidence that, in describing Brutus in Act 5, Scene 5, Antony says, "All the conspirators save only he/ Did that they did in envy of great Caesar" (69). This quote suggests that, apart from Brutus, all the conspirators overthrew Caesar not because they wanted what was best for Rome, but because they envied Caesar's power. Thus, it's important to consider that Cassius, like the other conspirators, is at least partly motivated by envy and the desire for power.