Do not confuse Obsessive Compulsive disorder with Schizophrenia.
if I said there is no justification for him doing that, I'd be just as wrong as the late 19th Century art critics who said there was no justification for Cezanne painting the way he did.
Many impressionists painted the way they did for many reasons: unusual imagination, defects in the vision, recreational use of ... whatever they were taking. Many had some mental problems. Their art justified their flaws. Their art will be here forever.
Brigg's art will be here forever too! Simply because it is well done mechanically, it is outstanding, unforgettable.
Whatever caused his imagination to go that sick, I cannot and shouldn't guess. However, what he created does not justify the cause.
Although ... I looked at it again, and you know, I see hate in his art. Yes, hate! So, if he, been a nice man (as Marcia said about him), wants to tell people in a nice manner how disgusting the mankind is, than I can see the justification of ... whatever his problem is.
Brigg even admitted this by himself; he said what influences his art is "an ugly wrinkle, fat neck, a pimple". Not a beauty of the human's body, not a deepness of the human soul, but the ugly inside of it.
Maybe he sees the beautiful in the ugly?