Julius Caesar was murdered in the Roman Forum to end his dictatorship and return Rome to a republic.
During his last military campaign against the Gauls, Ceasar had begun to express his dissatisfaction with how the Republic operated. He thought of it as chaotic and dysfunctional. Imperialism had reduced the power of the Republic, transferring much more power to the provincial governors who did as the pleased much of the time.
He hoped to strengthen the role of the central government and bring order to the corruption and weakness that permeated all aspects of the Republic. After crossing the Rubicon in 49 B.C. he defeated the forces of his opponent Pompey and used his newfound power to pass several reforms to strength his own executive powers at the expense of guild and the Senate. Many politicians and Julian opponents saw what was happening, but they were powerless to stop him. Some of his reforms were popular with the people, so he maintained a fair amount of popular support, which guaranteed him political power.
With no other options left to them, several Senators decided the only alternative was to assassinate him, end the dictatorship and revive the repressed Republic.
Gaius Julius Caesar was murdered in Pompey's Theatre where the Senate was meeting. This was to be his final meeting with the Senate before he joined his Army in Greece then preparing for a war with Parthia. The conspirators knew that once Caesar joined his army it would be impossible to murder him.
As to why ...
Did the Senators kill Caesar out of a hope of restoring the Republic and freedom to the people?
Or did they murder Caesar to safe guard their fortunes?
No doubt some acted out of nostalgia for an idealized Republic that never was, but it seems most had financial reasons for acting.
Like the Gracchi before him, who were murdered to stop their popular land reforms, so Caesar was murdered to stop his.
Now, some will say this is 'too simple' an answer, but we are talking about vast sums of money and huge tracts of land, land in fact that belonged to the people (ager publicus).
Caesar proposed to buy back the Public Lands, even though the landowners had no right to it in the first place, and at fair market value. This land would then be distributed to Veterans and to other citizens crowding Rome's streets and slums. He also would require that all farms employ not just slaves but Romans as well. The rich refused to give up any of their holdings and fought tooth and nail to keep all that they had.
One should also take note of the bias of our few sources, for although the Late Republic / Principate is one of the best-documented periods in Roman History, the gaps in our knowledge would fill the Alexandrian Library.
"In the highly skewed accounts of what is called history, Cicero, Brutus, Cato and the other oligarchs come down to us as the defenders of republican liberty; while Caesar - who tried to move against their power and privilege and do something for the poor - comes down to us as a tyrant and usurper.
And the people of Rome themselves ... come down to us hardly at all, or most usually as a disreputable mob. ... The common people of ancient Rome had scant opportunity to leave a written record of their views and struggles. Among the surviving primary sources, there exists little information on how the plebs urbana organized their collegia, and how they felt about wages, prices, taxes, wars, land policy, or employment problems.
Although we can draw certain inferences, history leaves us with only fragmentary impressions of their tribulations. Still ... what we know of the common people tells us that they displayed a social consciousness and sense of justice that was usually superior to anything possessed by their would-be superiors."
--From: The Assassination Of Julius Caesar - A People's History Of Ancient Rome by Michael Parenti, c2003, pp221-22