Zach was always a metaphor for bad parenting, BUT ... (SPOILERS!)

... now the metaphor has gone too far.

If you think about it this is basically a story about an absent father who leaves the upringing of the child to the mother. She, in turn, is an unsatisfied neurotic and fills the kid's head with toxic bullsh!t in his father's absence. Except now Zach is so screwed up that the metaphor doesn't work anymore.

Most kids can be manipulated into believing their fathers are scum, but not seeing them is key to the deception. A father has to be away most of the time for the mother to successfully convince their kid his dad is a bad guy.

Any kid who actually saw his mother try to physically kill his father would change his mind, or at least his response would not be anger at the father for defending himself. ESPECIALLY if he'd had plenty of opportunities to see what a tough time his mother was having keeping herself from killing HIM, too. I mean, in the series Zach has to snap her out of her bloodlust literally minutes before he chooses to set off the bomb.

So the only possible explanation for his reaction is either mental retardation (doesn't understand what's going on around him) or sociopathy (gets pathologically angry if things don't go according to his wishes and doesn't care how many innocent bystanders he hurts). It certainly can't be the Master's influence, since Zach does what he does with the Master already in the box.

And nothing we've seen so far suggests either of the two options. It's perfectly reasonable to think that Zach acts how he acts in captivity only to survive, and he actually leaves a clue for Eph when he's forced to move. But there's simply no credible explanation for his going full retard all of a sudden.