Earth is a crucible or a gauntlet which we have to endure to have salvation. Without the trial we would not know what we would have lost. If God did away with all external evil would evil still exist in the hearts of man. Consider original sin, some say a serpant tempted eve to eat the apple, but if she was tempted dose that mean the evil was in the serpent or in man? If man was only evil because of outside forces then eve would have told the serpant " I have one God, he commanded me to not eat of this fruit, be gon serpant". But she allowed herself to be currupted. Therefore the evil was in man the whole time.
It is also the reason all man kind suffers for that sin, we all would have eaten it.