At the end of Shakespeare's play, after the death of Brutus, Marc Antony, who would have no reason to praise his enemy, eulogizes Brutus, acknowledging that of all the conspirators he was the only one who had noble intentions:
This was the noblest Roman of them all.
All the conspirators save only he
Did that they did in envy of great Caesar:
He, only in a general honest thought
And common good to all, made one of them. (5.7.74-78)
The others, as Antony acknowledges, are envious. That Cassius is envious is glaringly obvious. In Act I, Scene 2, Cassius speaks to Brutus of Caesar,
...And this man
Is now become a god, and Cassius is
A wretched creature, and must bend his body
If Caesar carelessly but nod on him. (2.2.117-120)
He continues, describing a epileptic seizure that Caesar suffered, adding,
Ye gods! It doth amaze me,
A man of such feeble temper should
So get the start of the majestic world,
And bear the palm alone. (2.2.134-137)
Continuing his motif of Caesar being given too much power, Cassius, nevertheless, also continues to demonstrate his envy as he likens Caesar to a Colossus, who stands over the "petty men" like Cassius and Brutus who will be controlled, he implies, by such a tyrant. Casca, also envious, reinforces for Cassius that Caesar is desirous of power as he describes how Caesar pretended to refuse the coronet [crown] that Antony offered him because he acted as though he were reluctant to relinquish it.
Knowing that Brutus is not envious and is known for his noble nature, Cassius forges letters in order to convince Brutus that it would be honorable to assassinate Caesar. So, he tricks Brutus in order to serve his own desires.