How the existence of good and evil are explained in Zoroastrianism
All things good emanate from or are a part of God, while all things evil are part of Ahriman's plan and intention to assault or invade existence. God is not responsible for the bad whatsoever, nor is Ahriman responsible for any of the good. Whatever is good has its counterparts on the side of the evil one often.
God is all knowing but not 'all powerful' in their ability to completely prevent the evil one (Ahriman)'s intrusions into existence, thus why people suffer or experience bad and evil as such. Hence why they ask us to lend a hand in renovating existence.
Asha is defined as the ideal state of something in existence as most intended by God or the creator. Evil acts as an intruding force that blocks or prevents anything from reaching its state of best possible well-being.