If the AI doesn't have its own intention (perhaps having intention is a prerequisite for being truly intelligent), it's conceivable that we would program it with an intention or goal, such "find the most efficient way to produce as many paperclips as possible". If you assigned such a task to a human, it's fairly obvious that the task is subject to other things which are important to humans (for example, don't enslave or kill people to build paperclips, don't replace strip-mine all our farmland to find more paperclip material, etc.). But since an AI doesn't necessarily share, understand, or agree with all the implicit values we have, it's myopic view of the world could cause it to destroy us as a mere side-effect of trying to achieve it's singular goal.