There are many issues that lead to Caesar's death. To say that there was one issue is simplistic. Here are a few reasons (in no particular order).
1. There was a feeling that Caesar was transgressing Republican boundaries more than anyone else in the past. For example, Caesar became dictator for life (Feb. 44 BC). This was tantamount to "kingship" to some, a concept the Romans deplored! You should remember that the Romans expelled the kings in their past and established a Republic, where there was plurality and parity in power.
2. In connection to point one, there were dictators in the past, but the Roman reserved this office for only a short period of time for extraordinary circumstances.
3. Caesar's past also probably had a role to play. So, when he crossed the Rubicon, there was the beginnings of a civil war.
4. Caesar had the loyalty of the army. From this perspective, he had all the power and people knew it. He did celebrate four triumphs with great splendor!
5. Finally, he was planning campaign against Parthia. If he was successful, then what else? There was nothing for him to do. People probably feared that he would become a tyrant.
After beginning a civil war in around 50 B.C. that left Caesar without rival in ruling the world of Rome, Caesar began instituting a series of changes that centralized control of the Republic, as well as other reforms in Roman society and government. Eventually, he was able to assign himself the title "dictator perpetuo," the Latin term meaning "dicator in perpetuity," a development none too popular with the group of senators who assassinated him. The senators' attempts to return the Republic to the previous manner of operation backfired, however, with the onset of another civil war and the establishment of yet another dictator.
A simple answer would be that an insatiable desire for power led to Caesar's death. The senators who planned and carried out the assassination did so partly out of revenge (because they were followers of Pompey and believed that Caesar in his own quest for power had caused Pompey's death) and partly because they felt that Caesar might be crowned king and lead Rome back into a tyrannical monarchy, overthrowing the republic which sought to establish and maintain a balance of power.
An interesting theory about Caesar's assassination is that Caesar might have known about the plot and allowed it to happen because of the embarrassing symptoms of his epilepsy, because he knew that an assassination would immortalize him, and because he had carefully plotted out who would succeed him and how he would be remembered. This is simply a theory, but Caesar did dismiss his bodyguards shortly before he was killed, finalized his will, and gripped in his dead hand a note of warning (this was discovered in the recorded autopsy of Caesar's death).