Yes, it is true. Cannot give you scientific proof but that's what I understood on my spiritual journey :-) And it's not only truth about gods, but also about places, statues... for example. If you come to an Indian temple where people worship with pure heart and pray every day... and you are sensitive enough, you will feel how holy the place is, you will "feel the god" there. On the other side, if you buy a statue of a god, bring it at home, and if it will be for you just a statue from a vacation, it will not have that power that you feel in the temple. Whereas if you would pray to that statue every day, that power would be there. I am now at a point where I can feel these things - if I enter some space, it is very different if someone takes care of it spiritually or not. It's like cleaning, but on different levels :-)
There's also another New Age metaphysical teaching, represented e.g. by Teal Swan. And it perfectly fits to mythology as well. According to this teaching, there are a lot of beings that are interested to interfere with humans on Earth, since this is probably the most interesting experiment in the Universe :-D Some of those beings are here to help, some of them can be parasitic (like demons). People can have a lot of conscious and unconscious contracts with these beings and their power is basically proportional to the attention (conscious or unconscious) that they get from people.
According to the second part of your question, I think they cannot be killed. Anyone who would remind and remember them would again put them to life back again.