It seems that one should avoid question about cyber security tools (Since it is very opinion based. But is it allowed to ask about how specific popular tools work?), no code reviews, obviously no blackhat answers/questions and I think much more things which one should consider by asking a question. Is there any FAQ or some unwritten guidelines which I should follow? I totally agree about those guidelines but would like to know more.
You only got one down vote on your sqlmap question, and two upvotes, so I would not use that as an example of "downvotes"
There is a lot of guidance written - it's actually pretty extensive. We haven't collated it into a FAQ, but reading the posts here on meta should give you a very good idea of our stance on most kinds of questions.
So I'd suggest initially reading those posts in the Related sidebar to the right, and then just continuing to browse meta, maybe focusing on the acceptable-question tag initially.