SOMEBODY STOP THAT MADMAN! Where is truck-san, when you need him?
I see the drama unfolding. Dad was abroad all the time, sees something that isn't to his liking and starts to mess around without talking with Tarou.
My guess is, his own firstlove zombie turned into a brokenheart zombie (which is essentially a demon) and dangerous stuff happened to everyone, who was near him. So every fistlove zombie has to be shot on sight. Kinda like he had a dog, that turned Cujo and bit him. Than he returns from a business trip, sees that his son got a dog, takes a bat and clubs it dead.