We humans have all become foes of God by defecting from our proper goal.
Each of us has indulged an inclination toward evil deeds. Those who think about the human condition usually disregard or evade this fact.
For details see The Wickedness of Man.
The only plausible explanation of this fact is that mankind has abandoned the devotion to God that provides his proper ultimate goal, so that each of us begins his life as a wrongdoer, alienated from God and inclined towards negligence of our cultural task and violations of natural standards, distressed, heading for disaster, and with no tendency to change course.
For details see The Characteristics of Man's Defection.
Of all the classic literature of mankind, only the Bible gives this explanation.
If in addition to mankind there are other species in our selfkind, they are affected by the same defection. If there is any other selfkind, it may or may not also have defected; I see no way to guess.
"Similar answers to the basic questions about the human condition are found in plenty of other ancient sources."
Only the Bible says anything like this. Jews and Muslims may seem to accept it, but in fact they deny the full depth of man's universal wickedness. They consider that a man can redeem himself, usually by good deeds that make amends for the bad ones, or by finding goodness within ourselves. The Bible explains that there is no hope in such innate sources.
I leave open the question of how far this belief is implicit in the Hebrew Scriptures. Certainly it is only fully stated in the New Testament.
The Circumstances of Defection. On the question of the literal truth or otherwise of the Genesis story, see Adam and Eve.
The Cause of Defection. Nothing caused this defection, so the pseudo-question why it happened is meaningless, we can only confess that it happened. The relationship between God and man is not mechanical, it is (more but not less than) personal, and the words that best describe it are personal. Not every event is caused by previous events.
"It was caused by Satan."
On the idea that man's defection was instigated by a prior supernatural defection, see Satan.
The Forms of Defection. Defection takes many forms. See Idolatries.
The Consequences of Defection. Wickedness leads to all kinds of disaster, the natural consequence of ignoring our manufacturer's instructions. See The Consequences of Man's Defection.
"The human condition cannot be as bad as you say. If it were, there would be no hope for mankind, and despair would be the right attitude."
There is hope for man, but not from man. The next Section of this Creed explains.