The genius of the Lord's plan was letting Adam and Eve experiment, and when they made mistakes and ignored the counsels of God they would have a period of what we call "suspended judgment" which would give them time to recognize their mistake and repent and turn back before it was too late. Of course if they refused to recognize their mistakes there had to be an appropriate punishment so they would not repeat this same mistake in the future. Now it was this punishment of evil that Corianton had thought was unjust. Alma wanted Corianton to appreciate that the punishment was the vital key to the learning process, learning the difference between good and evil.