Well, he's a troon, he gets off on the power from violating people's boundaries and taking what rightfully belongs to others. Zach's proof to himself that he's a special, important, misunderstood genius is that he successfully stole her children's place in Heather life. We're all familiar with the kind of people who get off on being side ho/ side dick because it's proof to them that they're "winning" some kind of battle that the actual partner/spouse doesn't even know they're fighting. Zach's relationship with Heather is built around that impulse, but on steroids. The bond between mother and child is usually stronger than the bond between spouses, much to the chagrin of many a new father. For most women, when weighing the needs of her small child versus the needs of her adult spouse, the baby wins every time, no question. Hell, a lot of them can't stop acting like that when the "baby" is 25 and desperately needs mommy to butt out so they'll grow up and figure their shit out.
Essentially, Zach got Heather to "cheat on" the relationship between herself and her own children, and to continue doing it every single day. She continues to choose him over them- he gets to LARP being in college, so her kids will never get to go. He gets to pretend to be a gender scholar and famous porn star instead of having an actual job, so her children get a stroke-addled mother and a shoestring household budget. Instead of each child having their own private space, which is very important to teenagers, Zach gets an entire landfill/masturbation room. Since the kids have no privacy, Zach just walks in on them and blasts it all over the Internet. Every day he spends lolling around in the wank dump tweeting, he gets the validation of being chosen by Hagrid-Mommy in a way his own mother never did, but he's miserable, because it's never really enough. Heather is vastly inferior in every way to the real mom who didn't love him enough, and it doesn't matter how much he gets her to debase herself for him, because she was never worth much to him in the first place.
Nobody could generate enough validation to fill the hole inside him, and neither can Bad Dragon. The end of the story for at least 41%.