He brought blue prints of nuclear technology from his employer (Holland) without their permission, and developed Pak's nuclear crude bomb. Chinese also controbuted their might in Pak's efforts to build a bomb. Even today many of Pak's missiles are Chinese made, but assembled at Pak.
Pak is not a responsible State. Army coups are frequent, and fundamentalists have a say in each and every matter. Therefore it is a very dangerous bomb/technology in a dangerous/unsafe pair of hands. There are strong evidences to suggest that North Korea, Iran, Libya have all benefitted from Dr.Khan's (Pak's) blackmarekting of nuclear technology. North Korea, Iran, Libya all are very unsafe countries, like Pak. In a fit of anger any of these countries may drop the bomb, or terrorists in these countries may lay their hands on these and may drop in any place, which might kill lakhs of people and make the earth unsafe for living for many years.