The first hint of evil in the Tanakh seems to come in Genesis 3:1 (NJPS):
Now the serpent was the shrewdest of all the wild beasts that the Lord God had made. He said to the woman, “Did God really say: You shall not eat of any tree of the garden?”
The serpent is questioning God's instructions, which seems both unwarranted and irrational considering that everything in the world was created good. Genesis 1:31 (NJPS):
And God saw all that He had made, and found it very good. And there was evening and there was morning, the sixth day.
The odd thing is that the serpent was himself a created thing and so must have been good on the sixth day.
Generations later, God is prepared to wipe out humanity and all living creatures with it. Genesis 6:5-7 (NJPS):
The Lord saw how great was man’s wickedness on earth, and how every plan devised by his mind was nothing but evil all the time. And the Lord regretted that He had made man on earth, and His heart was saddened. The Lord said, “I will blot out from the earth the men whom I created—men together with beasts, creeping things, and birds of the sky; for I regret that I made them.”
What started humanity on a downward spiral from the "very good" of Genesis 1 to the "great wickedness" of Genesis 6? Was it questioning God's goodness?
About closing this question:
I've come to the conclusion that as interesting as this question is, it's off topic here. In my opinion, Genesis just doesn't say. Many commentators have opinions, but it seems to me to be just that: opinions. Ultimately, how you might go about answering this question says as much about your doctrine as about the text.
If you are still interested in this question, I suggest doing one (or more!) of the following things:
- Ask on Christianity.stackexchange.com
- Ask on Jewish Life & Learning
- Ask a more focused question here
- Ask someone whose opinion you trust
I've been struggling with this question for a long time and I find it somehow appropriate that it is closed—not because I am done struggling, but because there is no easy or satisfying answer. As Job says:
Job said in reply to the Lord:
I know that You can do everything,
That nothing you propose is impossible for You.
Who is this who obscures counsel without knowledge?
Indeed, I spoke without understanding
Of things beyond me, which I did not know.
Hear now, and I will speak;
I will ask, and You will inform me.
I had heard You with my ears,
But now I see You with my eyes;
Therefore, I recant and relent,
Being but dust and ashes.
—Job 42:1-6 (NJPS)