I'm strapped for time, but I'll try to give you a start to work from in chronological order.
Brutus was born to his mother, Sevilia, and Marcus Junius Brutus the Elder. However, Sevilia was known to be Julius Caesar's mistress as time went on. This meant that Caesar was regularly in Brutus' home, and was known to Brutus, who aspired to be like Caesar and viewed him as a father figure in his life.
As Brutus grew to be older, he strived for a position in Rome's government. His ambition and opportunist self rose quickly, especially with his connections. When civil war broke out between Caesar and Pompey (Caesar's rival), we see that Brutus sided with Pompey for political gain and fear for his life. This is our first clue regarding Brutus' betrayal. (It should be noted that Caesar gave orders to his army to spare Brutus and take him alive if he was captured. We can assume this was from his love to Sevilia and to Brutus too, as he did know him since his childhood.)
After Pompey was killed by Ptolemy of Egypt, Caesar pardoned Brutus with amnesty for serving on the opposing side of the civil war. Not long after this, Caesar gives the Brutus the title of Governor of Gaul, which Julius Caesar had recently conquered. About a year after this, Brutus is made to be praetor, for the purpose of drafting new legislation from Caesar.
We can only imagine that drafting Caesar's legislation cemented Brutus' care for him. However, the Senate grew increasingly fed up with Caesar. Brutus recognized that the Senate held a lot of power even though they were being controlled by Caesar. In his mind, Brutus served the Republic of Rome, and what it stood for. He did not this subscribe to this new form of Rome that was developing into an empire by the hands of Julius Caesar, who had declared himself from Consul to dictator.
For this reason, we see Brutus start to turn in his heart. He justified the dark motives of the Senate to get rid of Caesar in pursuit of a true Roman republic. As it definitely was changing. Brutus was a betrayer, though it was patriotic reasons. However, betrayal is betrayal, and he had a debt to Caesar for various reasons. The opportunist in Brutus got the best of him.
I hoped that helped. I tried to give you lots of background for inspiration.
