Which UFOs are Dangerous?

Most accounts by those claiming knowledge of visiting ETs [whatever they are] suggests that some ETs may be good or relatively friendly, and some ETs are bad, or dangerous, or evil. Also, if people are coming into contact with highly advanced crafts, they might emit dangerous radiation or have other effects. And since different ETs are apparently operating different types of crafts, the following question is suggested:

Which UFOs are the most dangerous either because of how they operate, or because of who/what operates them?

For example. I heard one claim that the mantis beings are very dangerous and that they fly one of the black triangle types of crafts – I think the solid equilateral black triangle with 9 or 12 lights on the bottom along the edges . So if you see that type of UFO, you want to avoid it if at all possible. And maybe you are fine to run out and greet a small saucer because they are operated by friendly gray aliens, as some legends describe for native Americans.