If the banishment decree imposed on Publius Cimber was the only reason to kill Caesar, then what happened there in Julius Caesar?
The suit of the Cimber brothers was only an excuse to surround Caesar, not the reason they killed him.
The assassins had been planning to kill Caesar for some time, since he returned victorious from the civil war with Pompey. They fear that he is taking too much power to himself, and they resent him for it. They do not want Caesar the dictator to become Caesar the king.
The assassin senators use the Cimber brothers’ suit to prove that Caesar is arrogant. He will not listen to the suit no matter how much they plead. Caesar has banished Publius Cimber, and he will not change his mind. He ignores Metellus Cimber’s plea, telling them that he never changes his mind.
I could be well moved, if I were as you:
If I could pray to move, prayers would move me:
But I am constant as the northern star,
Of whose true-fix'd and resting quality
There is no fellow in the firmament. (Act 3, Scene 1)
The suit to pardon Publius Cimber is just an excuse to surround Caesar. Brutus, Cassius, and the others all beg Caesar to listen, and Caesar is surprised. He does not understand why everyone is making such a big deal of this when he has already told them he has made up his mind.
To Caesar, it seems odd that they are all pleading. He is especially surprised that Brutus, who should know better, would kneel before him and try to intercede for Publius Cimber. At this point, Casca stabs Caesar, calling all of the others to him. They then all stab him, ending with Brutus. Caesar is shocked that he has been so betrayed, even by Brutus.
At this point it would be obvious to anyone watching that the whole Cimber incident was only designed to get allow the senators to surround Caesar without causing anyone to be suspicious. Since they were all pleading with him, they had an excuse to be there. It has the added benefit of making Caesar look arrogant, because he makes comments about being as constant as the Norther Star and as immovable as Olympus.