Plutarch and other historians all say the same thing: if anyone involved in the assassination acted in good faith and with high moral intentions, it was Brutus. He was uniformly considered to be idealistic (especially on the matter of Roman "republican" ideals) and "constant" -- the latter being a polite way of saying stubborn or stiff-necked. Brutus was the nephew and the son in law of Cato the philosopher (marriage to cousins was normal). Instead of pursuing a military career, which would have been normal for a Roman of his class, Brutus studied philosophy. He specialized in classical Platonism, but he was learned in all the contemporary schools of philosophy. Brutus' family had been friendly with Caesar, and Brutus' father had been proscribed and executed by Caesar's great enemy, Pompey. Nonetheless, for ideological reasons, Brutus fought on the side of Pompey in the civil war between Caesar and Pompey -- Brutus thought that Caesar was the greater threat to the Republic. Despite this, Caesar thought so much of Brutus that he gave orders that Brutus should be spared or allowed to escape in any battle.
After Pompey was defeated, Caesar pardoned Brutus and gave him several important government jobs, even leaving him in charge of Rome when Caesar went off for his foreign battles. Brutus was one of Caesar's most able administrators and was a well-liked governor. It was commonly thought that Caesar was grooming Brutus to be his successor. It was also widely believed that Brutus was Caesar's unacknowledged love child -- there had been a notorious liaison between Caesar and Servilia, Brutus' mother, at just the right time. Brutus was treated like a favored son, although accounts of Brutus "living in Caesar's house" may be allegorical.
Yet on the Ides of March, 44 BC, there he was, leading the other conspirators, drawing daggers against Julius Caesar in the Portico of the theater that Pompey had built a few years earlier, before he had been expelled from Rome and hunted down. Brutus had been convinced once again, this time by Cassius, who historians mostly agree was the most villainous of the conspirators, that Caesar, his benefactor and maybe his father, was destroying Roman Republican institutions. Brutus was trying to save the Republic, just has his supposed ancestor, Junius Brutus, had founded the Republic by slaying the last Tarquin king. Freud would have loved to get Marcus Brutus on his couch.
Caesar, according to the histories, was stabbed twenty-seven times. At first he resisted, but, according to legend, he accepted the blows after seeing that Brutus was among his assailants.
The Senators fled rather than staying around to hear the speech Brutus had prepared, but the next day, March 16, the conspirators, who styled themselves the "liberatori" (liberators), were pardoned and praised by the Senate. But their situation soured quickly. Later on the sixteenth, Mark Antony, who was Brutus' rival for the mantle of Caesar, gave an impassioned speech over Caesar's body in the forum, even exposing Caesars mutilated corpse to the mob. The mob seized the body and burned it in the center of the forum in a solemn hero's funeral.
Brutus, Cassius, and the others fled to Greece where they gathered an army in the hopes of returning to Rome and reestablishing the Republic. Octavian, Caesars' grand nephew and posthumously adopted heir, in temporary alliance with Mark Antony, defeated the liberatori in Greece, and first Cassius and then Brutus took their own lives to avoid capture and ignominy. By the end of 42 BC all of the liberatori were dead. It took Octavian another twelve years to secure sole rule.
Brutus' story is available in much greater detail on the Internet http://classics.mit.edu/Plutarch/m_brutus.html.
For Caesar's funeral and how Marc Antony's oration turned the mob against the liberatori, go to http://www.fordham.edu/halsall/ancient/appian-juliusdeath.html.
P.S.: 1) Plutarch is the main source for information about the assassination and its aftermath. He wrote his biographies more than one hundred years after the fact, but he had in hand several histories and biographies that were written by witnesses and even some letters from the conspirators (most of which have since been lost.)
2) Whatever their motives, the liberatori were essentially correct: Caesar was trampling all over Republican institutions. What they seemed not to realize was that those institutions had already been irretrievably broken by the civil wars that wracked the Republic in the decades before Caesar crossed the Rubicon. Julius Caesar was a symptom, not the cause, of the Republic's fatal illness.
3) Idealists or villains, the liberatori strategy stunk. You make a plan, gather your army, and then have your revolution. It really doesn't work if you try it the other way around.