https://www.jstor.org/stable/30072802?seq=1#metadata_info_tab_contents

M. Brutus's letter to Cicero, linked above, is decrying Cicero for supplicating the mercy of Octavian, not Julius Caesar, if that wasn't clear to you.

His anger at Cicero stems from his belief that this undermined his virtuous purpose in slaying Julius Caesar. Asking for clemency for Brutus presupposes both that M. Brutus was wrong to slay Caesar (though clearly that was intended... Antony deified Caesar and gave his actions the color of law, thus opening Octavian's path to power as the heir designated in Caesar's will) and that Octavian was legitimately vested with the power to decide M. Brutus's fate. In other words, because Cicero liked Octavian more than Antony, somehow Octavian's tyranny was legitimate where Antony's wasn't.

That is all--with the exception of the parenthetical--stated explicitly in the text of the letter. All things considered, he appears to be an idealist--he cast down Caesar as a tyrant, went to war with Antony for the same reason, and now rejects even attempts to secure him clemency with another tyrant who came up on the coattails of the tyrant before him.

M. Brutus touches on his motivation for killing Caesar in that letter as well--for the sake of liberty. He also expressed this in his speech to the public the day after the assassination, which is accounted in multiple histories of which you can take your pick. http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Cassius_Dio/44*.html#21

http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Appian/Civil_Wars/2*.html#120

http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Plutarch/Lives/Brutus*.html#18

Since this is a homework question, I will point you in a direction that you should explore further: Brutus was gens Junia, which had something of a reputation for tyranicide. Plutarch's Life of Brutus describes this legacy and its importance to Brutus (http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Plutarch/Lives/Brutus*.html#1) but you can get closer to the source by looking at Cicero, who frequently brought this up to and about M. Brutus himself (http://www.attalus.org/old/brutus1.html#53).

Lastly, M. Brutus minted coins both before and after the assassination. http://www.humanities.mq.edu.au/acans/caesar/CivilWars_Libertas.htm The coins after the assassination commemorated his act, but there were two coins minted long before that, the first commemorating his alleged ancestor through his father--the first Junii who ended the Roman Kingdom by exiling Tarquin and becoming the first consul of the Republic--and the second commemorating his alleged ancestor through his mother--S. Ahala, who killed Maelius for conspiring to make himself a king. This would at least seem to support Plutarch's and Cicero's narratives that Brutus was motivated by a sense of familial legacy to cast down tyrants.

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