When Shinichi yells at Murano it's not so much that he's upset at her as much as he's instinctively lashing out, out of fear that he is being infected by the Parasyte cells in his blood. Murano's innocuous "inner joke" questions then the validity of his very humanity.
All that of course, is a metaphor for going through a severe depression where you feel disconnected even while functioning perfectly in everyday life.
After all his "mother" did stab him in his very metaphorical and literal heart, and losing her decapitated remains to the sea would make anyone understand that a dead dog is not a dog anymore and a carcass means nothing.
Yeah, Shinichi is fucked up in the head, compared to what he was in the beginning.