Fuck man, I remember this show, I remember there was one about robot mutant dogs or something, I was about ten but I recognised the inherent shittiness of the whole set up even then. It was no Sliders.
I'm going to play the 'get inside the writer's head' game again and speculate how this plays out if I may.
The Onion Head guy can't be hit by weapons... Because he's a non corporal psionic projection of Asshole Cop! That's right, either intentionally or unintentionally Asshole Cop created Onion Head as an alter-ego and has been the murderer all along.