Essentially part of Tauheed (Oneness of Allah).
Aadal is the belief that God is Just.
He will reward or punish any person according to his deeds and thus the notion of predestination (where all decisions are God's and not any one else's) in one's deeds does not exist.
It is absolutely forbidden in Islam to believe that the Almighty, Merciful Allah planned our destiny and that the good and the bad are just His Will and there is no choice for us between them (God forbid!).
Those who ascribe to such lies do so because they want to blame their own evils on Allah and claim the good for themselves!
Although the attribute of Allah's Justice is not a separate entity of Allah for certainly the Almighty God can never be compartmentalized nor defined in any relative terms, it is nonetheless absolutely compulsory to believe and fully understand the importance of this attribute in Islam so as not to allow the evil suggestions of Iblees (curse of Allah be upon him and his progeny) and his companions from leading us astray.
How the God is Just
Unjust is defect, that cannot be in God.
Unjust is due to some need, God is not needy.
God has discarded unjust, then how He can be unjust?
God has announced that He is Just through all His Book revealed on the humans.
Just is present in everything He has created.
If we suppose that he is unjust then we cannot guarantee his truthfulness.