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.

Friends think I am too strong
saying this sentence...and here Zoraster says the same thing.
It is nothing but evil that blocks us or prevents us from union with God...
or makes us think we are separate or estranged from our Creator.
And it was not a mistake evil was portrayed as a snake...slithering and tricky.

I wanted to expand upon something, on the interpretation of what was given is mainly symbolic for. This is the more common interpretation and of what the original from the days of the Achaemenid Empire or earlier was.

It is also the duality of the good mind against the duality of the evil mind (Where all evil originates from), inside yourself and in others. Through this we know that evil is both internal and external. The free will to choose between the two in how you deal with yourself and with the existence all around you.

According to the consensus of the congregations who are part of the revival its said that Ahriman was overly personified later on to mean a literal being (Whom later may have influenced concepts associated with the devil in western country folklore).

So however, there are people in our community who still follow that interpretation, although they're in a smaller minority (The Sassanid and Zurvanite variants of the religious beliefs).

Adding to what I said you have this interpretation of what Zoroaster's message meant and the literal one, depending on which is most likely to have been.