If Caesar was such a brilliant general and wise leader, why did they kill him?
Because with his army he overthrew the Roman Republic and set himself up as dictator.
Shakespeare, taking off on Plutarch’s biography, makes him into a megalomaniac:
Vercingetorix Throws Down his Arms at the Feet of Julius Caesar (1899), Lionel Royer; Crozatier Museum at Le Puy-en-Velay (public domain photo)
“…I am constant as the northern star,
Of whose true fix’d and resting quality
There is no fellow in the firmament.
The skies are painted with unnumbered sparks,
They are all fire and every one doth shine,
But there’s but one in all doth hold his place:
So in the world; ’tis furnished well with men,
And men are flesh and blood, and apprehensive;
Yet in the number I do know but one
That unassailable holds on his rank,
Unshak’d of motion: and that I am he…” Julius Caesar, Act III, Scene 1
Shakespeare (and Plutarch) didn’t know Caesar but Cicero knew him well. This is how he summed up Caesar in his Second Philippic:
“His character was an amalgamation of genius, method, memory, culture, thoroughness, intellect, and industry. His achievements in war, though disastrous for our country, were none the less mighty. After working for many years to become king and autocrat, he surmounted tremendous efforts and perils and achieved his purpose. By entertainments, public works, food-distributions, and banquets, he seduced the ignorant populace; his friends he bound to his allegiance by rewarding them, his enemies by what looked like mercy. By a mixture of intimidation and indulgence, he inculcated in a free community the habit of servitude.”
That is why Cicero applauded when he heard they had killed him.
How did Caesar himself explain what he had done?
“I did not leave my province [and cross the Rubicon with my legions in defiance of the Senate] with intent to harm anybody. I merely want to protect myself against the slanders of my enemies, to restore to their rightful position the tribunes of the people, who have been expelled because of their involvement in my cause, and to reclaim for myself and for the Roman people independence from the domination of a small clique.” Civil War, Book 1, 22
By “a small clique” he meant the Roman senators.
But once he had established himself in Rome, he never took any step to restore the Republic. On the contrary, he had himself proclaimed dictator for life and some believed he even wanted to be crowned king. That is when his opponents decided to assassinate him.
The Death of Caesar by Vincenzo Camuccini (public domain photo)
See what has been said in his defence in Why Did They Kill Caesar 2