The Roman Republic was broken in many ways by the time that Gaius Julius Caesar came along, but it was based on a constitution, checks and balances of power, and the votes of the people of Rome. If someone took on the role of dictator--a sort of super magistrate with special legal powers--it was only an appointment for six months, one which most men resigned from early.
So it's not terribly difficult to imagine a group of senators being driven to murder when Caesar came along and had himself declared dictator in perpetuity.
Setting aside all that the Roman Empire did after the Ides of March and everything else that the assassins couldn't have foreseen, how ethical was what they did? At that point, was assassinating Caesar justifiable or not? If not, can any political situation ever justify assassination?