Roman senators plotted to kill Julius Caesar because they feared that he would declare himself Emperor. Given that Rome had already had a history with monarchy and tyrannical foreign kings, their fear could be said to be justified. More practically, the Senate was also the main source of power (politically and militarily) in Ancient Rome and they were rightly concerned with losing that influence, as is often the case with such influential leaders.
The senators' evidence that ultimately convinced Caesar's close friend, Brutus, were Caesar's actions following his conquests in Gaul and his crossing of the Rubicon in pursuit of Pompey during their conflicts following the end of the First Triumvirate. This perceived flagrance in flouting the traditions of the Roman Republic (no army was allowed to enter Rome) rightly concerned many as well.
Caesar's knowledge of the people and what they wanted made him a very charismatic dictator through his public works projects, festivals, and event games (Panem et Circenses) designed to gain their support for his other initiatives. In a time in which great men shaped the destiny of Rome, it is one of history's great mysteries what Julius Gaius Caesar would ultimately had done: would he have brought renewed greatness to Rome or put his own ambitions first?