Caesar was indeed ambitious. If he was ambitious, why did he refuse the crown offered to him by Mark Anthony? It is all part of his plan. If it looks like Caesar does not want to be emperor, the people will want him more. He was already dictator perpeutuus. The crown was just a formality.
One of the indications of his desire for power is to look at how he refers to himself in the first person. He elevates himself above everybody. He doesn't say "I will go forth". he says, "Caesar shall forth".
In his desire for power, he attends the Senate against all omens warning him not to go.
Did he need to be stopped? If Roman was to continue as a Republic, yes. Was the the way to stop him to kill him? Assassination is never a good solution.
The Republic was already dead, or least on its last breath, thanks in large part to Sulla, and by an even greater degree to the greed of the Senators.
The assassins were not looking to restore "freedom to the people" but rather to safeguard and enhance their own wealth.
Caesar was pushing forward land reforms that would restore the public lands (ager publicus) to the people. The Senators had taken these lands for themselves, farming them with slaves. Caesar sought to grant this land to Army veterans and Roman citizens as a way of re-leaving the crushing unemployment in Rome. He offered to purchase the land from the Senators at market prices even though the land did not belong to the Senators in the first place. Additionally he required that a certain percentage of farm workers be freeborn citizens and not solely slave labor. Both of these measure would cut deeply into the Seantor's income.
There is also the question of jealousy. Romans were fiercely competitive, and the system of honors and offices was designed to prevent any one person from out shining his fellow senators by too much. However this system was proving inadequate against the onslaught of talent that characterized the Roman leaders of the 1st century BCE. Caesar's achievements simply blew the system out of the water.
The real question is not "Did Caesar need to die?" but rather "What did the assassins accomplish by murdering Caesar?"
What they achieved was not what they hoped for. Indeed, their act made possible the rise of Octavian (Caesar's grand nephew and adopted son) who, although not as physically brave nor as militarily or artistically gifted as Caesar was, never the less, a shrewd politician who was able to accomplish was Caesar could not -- a system of government that reconciled the Romans and the world at large to the reality of the Roman Empire.
See: The Assassination Of Julius Caesar A People's History by Michael Parenti c2003
Caesar Life Of A Colossus by Adrian Goldsworthy c2006