The original text
The tenses used in the quoted passage in another question ("Would", meaning "possible or likely") gave me a little trouble trying to follow the scene. Being curious, I searched the web, and I now believe that your text was adapted from Chicken Soup for the Soul: Unlocking the Secrets to Living Your Dreams, page 124.
Here is the original text, with the differences highlighted:
There was a time in this country when you'd be considered a jerk if you passed by somebody in need. Now you are a fool for helping. With gangs, drug addicts, murderers, rapists, thieves and car jackers1 lurking everywhere, why risk it? "I don't want to get involved" has become a national motto.
1The original uses "car jackers" though carjackers is the common usage.
Is the for in "Now you are a fool for helping." used to show a reason or cause here?
Yes, it's used to show the cause (helping) of the reaction (being a fool). According to Practical English Usage by Michael Swan, entry 207.3,
207.3 causes of reactions
For ...ing can also be used after a description of a positive or negative reaction, to explain the behaviour that caused it.
We are grateful to you for helping us out.
I'm angry with you for waking me up.
They punished the child for lying.
He was sent to prison for stealing.
Is it better to insert "or/and" between the "rapists" and "thieves"?
Although it is possible to omit and, we don't do that very often. In my opinion, it's better to keep and there. And, obviously, in the original there is an and between thieves and car jackers.