The wildest theory I have heard is that the grey ETs are demonically possessed but can be repelled by invoking the name and power of Jesus Christ. More measured if still cautionary views are offered by French scientist Jacques Vallée, who describes UFOs as "messengers of deception," and history professor David Jacobs, who sees UFOs as here for their own purposes, which, pursued without our consent, are almost certainly contrary to our interests. Others distinguish the so-called Nordic ETs are genuinely friendly toward humanity.
Some theorize that ETs are from a future version of humanity or a breakaway civilization that originated on Earth ages past. Then there is Jim Sparks, who claims to have begun as a resistant abductee to a cooperative one due to the ET message that we are killing the Earth because of our destructive practices toward the environment.
The UFO/ET subject is fascinating but offers little in the way of solid evidence. Mundane, day to day pursuits are in the end more compelling.