In short, Caesar's ambition is the impetus of the assassination plot. His desire to become king is believed by the group of would be assassins to be an imminent threat to the current way of life. Caesar's rule would overturn the republic, and Cassius concurs that Caesar is treated like a god though he is merely a man.
the conspirators kill caesar for romans, as they are afraid of caesar to be king and to be powerful .
brutus kills him although he is friend to caesar,as he thinks that caesar will be tyrant so he should die for Rome
The conspirators felt that Julius Caesar is too ambitious and must be assassinated for the welfare of Rome. They were jealous of the prestigious and undeserved status that was bestowed onto him, which made him look like a "holy god" even though he was just a mere human being.
Julius Caesar might also be at fault in this assassination even though he was the victim. The conspirators sent him a letter regarding his assasinationa and death, but Caesar dismissed it as crazy, believing that his public self is so strong that his private self cannot be harmed.