The big problem that Caesar's "friends" and others had with him was that they were jealous of him. Their own personal insecurities caused them to look at Caesar as a threat, rather than as an ally or colleague. They were afraid that he would gain overwhelming power, as well as benefiting financially. The reasons for the betrayal of Caesar and his resulting murder are not unusual; people have been power hungry since time began. Even Caesar's closest friend, Brutus, allowed himself to be convinced that Caesar was a threat and participated in his murder.
Julius Caesar was attaining too much power. After conquering Gaul and bringing back much wealth, most of the Romans loved and adored him. But some didn't. Caesar later assumed power as the "dictator for life" and here lies his problem. According to a primary source biography on Julius Caesar written by Plutarch, “the most open and deadly hatred towards him was produced by his passion for the royal power”. Even some of his close friends, including the well-known Marcus Brutus plotted against him. Thus, on the Ides of March (March 15), 44 BC - Julius Caesar was stabbed to death. Famous words were spoken on that faithful day: "En tu Brute? Then fall Caesar". So, all in all, Caesar was killed because people hated his passion for power, jelousy, and a few mistakes here and there (e.g. Disobeying the senate's orders to disband the army after the capture of Gaul).