I'm sorry if this is offending but think of it like this:
If you see a bear, the first this you'll do is probably get scared right? Or maybe even run. It's human instincts. So as Ajins, They were still under investigation on this manga, It's also like a virus since it said that the Ajin will not know that they are Ajins unless they die. It's actually a 50/50% for both human side and ajin side but there's just a 5% more on the humans since they use ajins as experiment subjects without considering the Ajin itself. Even if it's for the sake of the Humans, Ajins are also once humans and i think the humans still did not know that even if ajins become immortal, they can still feel the pain of dying