How much of a jerk is Hahp's dad? We're not even really sure where to start.
First, Hahp's dad clearly abuses Hahp and his mother. Like when Hahp's mother starts to protest about something small, she "started to say something to him. He lifted one hand and she lowered her head" (2.16). Obviously this isn't the first time he's raised a hand to her in anger or to discipline her. As Hahp remembers: "She stood up for me against him sometimes—and she always paid for it in bruises" (6.6). This guy keeps his family in a constant state of fear.
As if that's not bad enough, Hahp's dad has a little something on the side of his marriage. Hahp finally puts the puzzle pieces together and realizes that he's seen Celia (one of their servants) emerging flushed and disarrayed from his dad's chambers while his mom weeps (26.10). So we can add marital infidelity to the list of ways in which Hahp's dad is not a nice guy.
He also seems like he's nasty when it comes to finances and social status. Hahp thinks about how another family, the Abercromes, produced several wizards: "I knew this because my father had told me, his voice bitter. They were known for their massive wealth but were several generations past the ones who had actually earned it. It was old money and very respectable. My father hated them" (8.4). Lots of people struggle with jealous, but hating folks who have what you don't is a whole different ball game of evil.
More ways in which Hahp's father is unpleasant: he constantly points out that his son is "slow-witted and a coward" (8.6). When Hahp's father and mother leave him with the wizards, Hahp notices: "He did not look back at me, and he kept my mother from giving me so much as a glance of farewell" (8.14). Gee thanks, Pops—so much for saying goodbye to your son before he enters an academy that could very well kill him.