Apparently you haven't yet read the play. We learn at the very beginning that some members of the Roman senate are becoming fearful and envious of Caesar's growing popularity with the people. They believe he will do away with the republic, and the senate, and declare himself king. Several of the senators, especially Cassius, try to win others to their side. Their prime target is Brutus, who was very close to Caesar (possibly even his illegitimate son). When they eventually do win Brutus over, the conspirators plot to assassinate Caesar. Their plan comes to fruition on March 15, the Ides of March, about which the soothsayer had warned Caesar.
Now, go read the play!
Caesar is assassinated by members of a conspiracy who believed that Caesar was far too ambitious to rule Rome well. Even his friend, Marcus Brutus, was convinced to join the conspiracy because Brutus valued his country above his friends and family. Caesar was lured to the Senate House with the idea that the people were going to offer him the crown. Even though Caesar had previously made a public display of refusing the crown when Marc Antony offered it to him three times, Caesar was still ambitious enough to have accepted absolute power if it had been offered again. The conspirators stabbed Caesar "three and thirty" times in the Senate House.
He was stabbed 33 times by the conspirators at the Capitol. I hate to nag, but it's impossible to have actually read Act 3.1 and not know this.
To put it bluntly, he was stabbed while on his way to the capitol. Furthermore, he was stabbed by his best friend Brutus who was part of a conspiracy against Caesar becoming dictator. Of course their is a much larger story behind this, that's why you should read the play.