Why would a time machine present a possible danger for humanity?
So far as we know, time machines only exist in science fiction literature and will never exist in the real world. However, if they could exist, they would potentially be dangerous to humankind. They would be more dangerous to individuals, but they could be dangerous to all of humanity as well.
They could be very dangerous to individuals because it would be very easy for time travelers to inadvertently cause some event in the past that would alter the way our personal histories unfolded. For example, my father was Filipino and lived in the Japanese occupation of the Philippines during World War II. He could easily have been killed by some random act of an occupier or by a bomb dropped by the Americans or from any number of other things. A time traveler might have caused this to happen, even without meaning to, leading to me having never existed.
On the scale of humanity as a whole, there is less danger. It is unlikely that anything would be done in the past to completely destroy humanity. Of course, there is the chance that someone could travel back in time and help Hitler win WWII, thus leading to a much worse world today. But it could just as easily happen the other way and someone could go back and kill Hitler before he could cause the Holocaust.
Luckily, all of this is completely speculative and so we do not have to worry about what could happen due to time travel.
According to Kant, Ouspensky, and other pjilosophers, time does not exist in the universe (ding an sich), but is simply an ordering, taxonomizing device built by the "I am", just as space does not exist in itself but simply as an ordering device ("three dimensions", for example, is a constructed mathematic device for describing objects in space). Does your closet, for example, have an ordering device, or do we simply divide our clothes according to some convenient system--winter clothes, shoes, sweaters, etc.? We "go backwards in time" every time we remember an event--that event isn't somewhere in space--it is in our mind, and we give it the name, "the past". I know this is a difficult idea to grasp, but there are only two pieces of "data"--the "I" and the "Other". Many philosophers have tried to address this concept, Plato among them, but most famously, Descartes, and Martin Buber, and the clearest of all--Ouspensky. Someone once remarked, "Time is an invention of the mind, so that everything doesn't happen at once."