My own research into ways dangerous AI systems might emerge suggests that the science fiction trope of AIs and robots becoming self-aware and rebelling against humanity is perhaps the least likely type of this problem. Much more likely causes are deliberate actions of not-so-ethical people (on purpose), side effects of poor design (engineering mistakes) and, finally, miscellaneous cases related to the impact of the surroundings of the system (environment). Because purposeful design of dangerous AI is just as likely to include all other types of safety problems and will probably have the direst consequences, that is the most dangerous type of AI, and the one most difficult to defend against.