I cannot agree with the above post that Caesar was assassinated because of his rejection of kingship; in fact he was assassinated because it was feared that he would proclaim himself King.
Caesar's greatest accomplishment was not so much his conquest of Gaul, which of course was significant; but his ability to secure rule of the entire Roman Republic for himself and to transform it into the Roman Empire. During the last days of the Republic, when Rome was racked by civil war, Julius Caesar raised his own private army using his own funds. In addition, he spent a great deal of money providing entertainment with the common folk, primarily gladiatorial fights and fights with wild animals. This made him immensely popular with the common people, and earned him membership in the First Triumvirate together with Pompey and Marcus Crassus. Caesar was to remain on the far side of the Rubicon River and leave Rome to Pompey (Crassus was killed during a slave revolt.) However, he was considered dangerous because of his popularity, and an attempt was made to remove him from power. In response, he marched on Rome with his army and took control. Pompey fled to Egypt.
Caesar is said to have muttered Jacta alia est ("the die is cast" when he crossed the Rubicon. Since that time, one is said to have "crossed the Rubicon" when one has reached the point of no return.
Caesar ruled under a provision of the Roman Constitution which provided for the creation of the office of Dictator during national emergencies. He married Cleopatra, Queen of Egypt, and set up colonies in Gaul, North Africa and Spain which he populated with veterans and poor folk so that they might have a new beginning. His popularity, however, made many fear that he would have himself crowned king--something the Romans never had--and he was accordingly assassinated.