Casca describes Caesar refusing the crown, and Brutus describes Caesar as ambitious.
There are actually very few positive references to Julius Caesar in this play. Most of the time he is discussed, it is with a negative attitude. This is because the play is really about the assassination of Caesar and the aftermath of that. The assassins spend a lot of time discussing why they feel that Caesar should be assassinated and then explaining why they did.
A good example of a negative view of Caesar is Casca’s scoffing depiction of the incident with Antony and the crown at the Feast of Lupercal. Brutus and Cassius are having a discussion (where Cassius is trying to convince Brutus to join the conspiracy), and do not see the event. All they hear is cheering. Casca later describes what occurred, and his description is not positive to Caesar. Casca sneeringly describes how Antony offered Caesar a small crown three times, and Caesar refused it. You would think this would be positive, but Casca sees it as a manipulation of the crowd.
[The] rabblement hooted and clapped their
chapped hands and threw up their sweaty night-caps
and uttered such a deal of stinking breath because
Caesar refused the crown that it had almost choked
Caesar; for he swounded and fell down at it: and
for mine own part, I durst not laugh, for fear of
opening my lips and receiving the bad air. (Act 1, Scene 2)
Casca is describing Caesar’s fit, perhaps from epilepsy. Although Caesar was really sick from something at this time, clearly Casca sees the whole thing as a publicity stunt. For whatever reason Antony offered Caesar the crown, Casca is not appeased by the fact that Caesar refused it. He still thinks that Caesar is ambitious and dangerous, and he is still Caesar’s political enemy.
Of course, Brutus is the one who describes Caesar in the most destructive way, during his speech to the crowd in which he explains why he and his conspirators killed him. It is in Brutus’s best interest to make Caesar look as bad as possible. He and his fellow senators are patriots and not murders, and they liberated Rome, the way he tells it. Caesar was taking over, and had to be stopped.
Had you rather Caesar were living and
die all slaves, than that Caesar were dead, to live
all free men? As Caesar loved me, I weep for him;
as he was fortunate, I rejoice at it; as he was
valiant, I honour him: but, as he was ambitious, I
slew him. (Act 3, Scene 2)
Brutus does not mince words in his speech. He never denies killing Caesar. He is proud of it. He is trying to convince his fellow Romans that Caesar deserved to die. In Brutus’s mind, Caesar was already a king in everything but name. He was too dangerous to be left alive, and they had to do something before he became too powerful and it was too late.
After Brutus speaks, of course, Antony stands up and tries to undo the damage by showing the crowd Caesar’s will—and his body. He paints Brutus and the others as murderers rather than liberators, and he tells the frightened Romans that Caesar was looking out for them and loved them, and that he, Antony, loved Caesar. It works. The crowd turns on Brutus and the other conspirators, and Rome is his (for a time).
Julius Caesar’s legacy was a complicated one, even in his death. Shakespeare presents it as such. Brutus is not wrong when he says that Caesar was ambitious. However, he did not want to be king of Rome. Such a thing was an anathema to the Roman people, and Caesar was Roman through and through. He is the true patriot. For all of the arrogance with which Shakespeare portrays him, which he does seem to indeed have possessed, Caesar seems to really have loved Rome and wanted to make it stable and functional. He never does anything over the course of the play to contradict this.