In scene 3, Cassius, who has asked his bondsman, Pindarus, to stab him, says the following before he expires:
Caesar, thou art revenged,
Even with the sword that kill'd thee.
It is clear from his final words that Cassius felt that Caesar's murder had to be avenged and he, being one of the assassins, had to be punished for his heinous act. For Cassius, there is honour in his own death. He has atoned for his crime and recognises the irony of dying by the same sword which he used to slay his emperor.
In scene 5, Brutus asks Strato to hold his sword so that he may run into it, killing himself. Brutus' final words are:
Caesar, now be still:
I kill'd not thee with half so good a will.
Earlier in Act 5, Brutus had been visited by Caesar's ghost and in his dying moments he refers to Caesar's restless spirit which can now be at peace since his assassin has now also lost his life. In the second part of his statement, Brutus alludes to the torment he had experienced in murdering Caesar. He had been Caesar's confidant, trusted by him and loyal to him. Brutus had felt that he had acted for the common good, persuaded by Cassius and his co-conspirators that Caesar's death was in Rome's best interest. Brutus' words reflect regret as he states that there had been a greater desire (will) in him to kill himself than there had been to kill Caesar. His conviction then had been half of that which he has now had.
Yes, both men had a choice. They could have battled on and would have either been captured or killed. However, there would have been greater dishonour in both options. Suicide in the face of defeat was, for a Roman soldier, a greater honour. In this sense, then, both Brutus and Cassius felt that they had no choice but to kill themselves.
Brutus states that it is easier for him to run on his own sword than it was to kill Caesar.
There is much to be said for this statement. Brutus insists that it was with great difficulty that he killed Caesar. So why did he kill Caesar if it was so difficult.
Brutus fully believed that Caesar had become overly ambitious.
In his death, Brutus had no choice. He could not serve under leadership that supported Caesar in all that he had become.
Brutus could not submit to Antony or Octavius. That would make his life and purpose to be in vain. He had no choice but to commit suicide.
He died in honor of his belief that Caesar had become power hungry.
He died with his purpose being fulfilled. He had rid Rome of the overly ambitious Caesar.
Sadly enough, Brutus and Cassius admit that Caesar has all power even in death. He is revenged for his death when Brutus and Cassius commit suicide.
Caesar can now rest in peace. "Caesar, now be still," in the words of Brutus before he fell upon his own sword.