Why did they kill Caesar?

Answer 1

When human motivations are involved, "Why?" is the most difficult question to answer, and a question which some historians do not attempt to answer. According to the play by Shakespeare, the senators were motivated by a misplaced sense of patriotism. I think it more likely that they feared for loss of their power and possibly their lives. The previous dictator Sulla had no respect for a Senatorial toga, and he ordered Roman senators killed as easily as he ordered the deaths of other men. Sulla violated Roman law by entering Rome with his army, and Caesar had done the same. After the establishment of the Roman emperors, the power of the Roman Senate was nominal at best. The paranoid Tiberius had senators executed on the flimsiest of charges, and the insane Caligula opened a brothel in which senators' wives were required to serve as prostitutes. Even the soldier-emperors like Septimius Severus and Aurelian, who administered the Empire very well, had little patience with the Senate. The Roman Senate continued to meet after the last Roman emperor was deposed in the 5th Century, but it had no power outside the city of Rome itself, until it was finally abolished by one of the Germanic dictators in the 6th Century.

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The Senate aristocrats killed Caeasr because they perceived him to be a popular leader who threatened their their priviledged interests. So therefor they thought that he was going to go against their word.

Another View:

The government of Rome had broken down when opportunistic generals relied on their soldiers and ex-soldiers to back their ambitions. Dictator Sulla tried to establish a counter-balance to the senatorial class by transferring much of its power to the equestrian class. This rebalance might have worked but when he retired after a couple of years, things reverted to usual instability, with the opportunists re-emerging, contesting for power.

Then Caesar re-established control and tried to do better than Sulla by getting declared Dictator for life. A very shortsighted policy: it was easily circumvented by the ambitious ones terminating his life.