Does a person bear the sin of one whom he taught the internet, and who then deviated by using it to access pornographic websites? And is he considered to be withholding knowledge if he refrains from teaching the internet for fear of deviation?

The basic principle is that learning and teaching about the internet are permissible, unless the learner or teacher intends to use it for something forbidden by Islamic law, such as accessing pornographic websites. In such a case, it becomes forbidden due to the cooperation in sin and aggression.

There is no sin upon you before you knew the intention of the one you taught. However, you must cease teaching him once his intention becomes clear to you. If you do not teach anyone, it is not considered withholding knowledge, because the evidence regarding the prohibition of withholding knowledge pertains to religious knowledge.

Withholding internet access—despite the need for it—from someone who intends to use it for a permissible matter in his religious or worldly life is not without sin, if that person is the only one who can provide it and no one else is available. However, withholding it from someone who intends to use it for a forbidden matter is obligatory.