Dr. Hawking's statement "Aliens are dangerous" is a valid statement and basis of an argument upon the existence of space faring intelligent alien life that has developed technology throughout the course of their evolutionary existence.
Not only is the statement valid, but it is also rational. Paraphasing "to my mathematical mind a discussion is logical" on the existence of aliens somewhere in the galaxy or the larger universe, because he is considering the plausibility of aliens that have progressed along a technological route of their society to become a space faring society. Furthermore, it is rational to consider their existence based entirely upon our own existence and current state of technology that has allowed humanity to travel beyond our home world to our neighboring Moon and our exploration of the planets in our solar system.
Yet, it is irrational because humans do not really have a definitive example of an Alien species from another world or solar system; all that humanity has is a preconceptual idea of what an Alien might physically be like. Unknowingly, humanity is trying to describe and define the proverbial Elephant all the while blind to the unknown existence based on the plausibility.
Therefore, to express the idea and promote the concept that "Aliens are dangerous" without fully knowing if Aliens are dangerous, as some may not be, is thus an irrational fear based upon unknowns.
Yet, there is more to Dr. Hawking's statement and that would include humans and our very essences of further evolutionary development beyond the Earth. What if "We" become the "Aliens"?
If a group of humans colonist decide to move to Mars in the later half of the 21st Century and exist for 50,000 or 200,000 years genetically separated from Earth; the plausibility of evolutionary changes to the genetic sequences might thus give rise to a new species of humans. Hence, they would become Aliens. His argument that the Aliens are aggressive with their technology use, is even valid given that humans have aggression and our entire prehistory and history is full of wars and destruction.
Oftentimes, within a simple statement there is rationality and irrationality that occurs. It is often surrounded by valid and validity arguments and debates.
Is he actually referring to the plausibility of and the existence of extraterrestial aliens, or is he foreshadowing the future evolution of humans in extraterrestial environments and what they would be capable of if they were to discover a primitative society that is just beginning to develop their own space technologies?
Even the psyops discussion takes one into the shadow lands of the unknowns and paranormal.
Our species has existed for 200,000 years without the discovery of an extraterrestial alien species. Although one could equally argue that humanity has discovered terrestial alien species as they moved out of Africa into the various climates and ecological regions of the world, to the demise of various other species. However, only one or two actually poised a potential threat to their existence: H. neanderthalus and H. florensis, both of which are now extinct. Our species could exist for 200,000 to 500,000 more years without ever contacting or having a "First Contact" with an extraterrestial species or a message from some extraterrestial species could be discovered tomorrow.
However, Dr. Hawking has stated "Aliens are dangerous" and the implied meaning is they are a potential threat whether they exist or not. In our fears we progress from the unknown to the known then to a new relm of the unknown.