Why did they kill Julius Caesar?

Julius Caesar was killed because he was arrogant, and the conspirators felt that he had too much power already and was trying to gain more.  They were afraid he would name himself king.  A group of powerful senators assassinated him in the senate in 44 BC.

Caesar had an interesting career because he came of age during a turbulent time for Rome.  His was not the first regime to be volatile or bloody.  Rome had already been rocked by Gaius Marius and Sulla's power struggles.  His first real power was when he joined with Pompey and Crassus in the first triumvirate.  Pompey was a military man and Crassus was a money man.  Caesar was the politician.  Caesar's alliance with Pompey was not to last.

Caesar actually had many talents.  He was enormously successful in his military campaigns and made himself plenty of money.  He also made money for Rome.  In this way, he gained himself money and followers.  The army was particularly loyal to him, especially when he made them rich.  This is why they agreed to follow him when he marched on Rome to snatch it from Pompey’s grasp.

Marching on Rome was illegal.  It was this bold act that made the senate feel threatened.  They began placating Caesar.  They had statues erected.  Caesar was named dictator for life.  The senators did their best to tolerate Caesar’s mopping up actions after the war with Pompey, including those that put him in Egypt for months and left Mark Antony in charge in Rome, which annoyed many of the senators because they did not consider him qualified.

The conspirators' biggest fear was that Caesar would declare himself king.  Although to us the role of "dictator for life" might seem worse, to the Romans there was nothing worse than a king.  They considered their republic evolved from having kings, and that having a king was having a tyrant.  Caesar continually insisted that he did not want to be king, but they did not believe him.  

The senators of Rome decided to prevent Caesar from becoming king and took an unusual and extreme action.  We do not have all of the details of the assassination, but we do know that there were about two dozen influential senators involved, including Brutus, Cassius, Casca (who knew how to wield a dagger), and Decius Brutus (who was close to Caesar).  They hid their daggers and came up with a pretense for attacking him, and then attacked him right there in a senate session.  He was stabbed 23 times and died of his wounds.  His death left Rome in a turmoil and another bloody civil war which eventually ended with a triumvirate of Antony, Octavius Caesar and Lepidus in charge.