I’ll try to give you some tips for creating a post apocalyptic helmet, or helmet in general. A helmet is like a face, it gots also a facial expression. Very small changes can influence the entire look, the same as with your face. Lower eybrowns and your whole expression changes. Your brain tries to recognize known elemets of a face in your helmet. So you got also something like eyebrowns, eyes, nose and a mouth. If you want to create an evil expression try to change the “eyebrowns” first.
Here you see that the first picture has a more neutral expression than the others.
the same here, from no eyes, over silly to almost cool and finally cool & evil. The second picture has also a too long and big face.
Having no mouth is ok, but if you add additional parts on the lower it can be recognized as a mouth. Like on the last pic, it looks like an open mouth…which is bad.
here are some examples of our post apocalyptic helmets and masks