Was he trying to get his family to be better humans but going about it the wrong way or was he just a nasty human being?
In relation to this answer I think papa's intentions where to obviously want the best for his family (wife and children), this is a good thing although it is only one aspect. Another aspect are the ways ways he went about accomplishing his aims in life.
Papa is a complex character as he is deeply religious and God fearing, "Because God has given you so much, he expects much from you, he expects perfection.” He says. He demands perfection from his family probably because he feels he is taking the role as God in his household; one example of this is when Kambili ranks second at school and is terrified for when the time comes to tell Papa as she knows she will probably get punished. Kambili says “I waited for him to ask Jaja and me to take a sip, as he always did. A love sip, he called it, because you shared the little things you loved with the people you loved... The tea was always too hot, always burned my tongue, and if lunch was something peppery, my raw tongue suffered. But it didn’t matter, because I knew that when the tea burned my tongue, it burned Papa’s love into me.” Kambili didn't mind that the tea always burned her tongue because it "burned Papa's love into her" and also because of course she could have been punished if she decided not to. Kambili loved her father but also feared him, Papa took the role as God into his household. Kambili could have mentally merged God and Papa together in her mind and this resulted in the utter respect and obedience she provided him with.
The fact that Papa managed to keep his teen aged children so anti-rebellious is a positive because it contributes to Kambili and Jaja's success at school and in general, which any parent would be extremely content about, but the negative in this situation where the extreme punishments he came up with, such as making his children put their feet in boiling water as a result of "walking in to sin".
Papa had decided to not have anything to do with his own father - Papa Nukwu, and his reasons for this where that Papa Nukwu was not a catholic and still lived on ancestral traditions and beliefs which Papa sees as evil. The question here is do we blame Papa? Colonialism in Nigeria offered Papa the privileges of access to higher education and grace, but at the same time colonialism enslaved the Igbo people and made their traditions significantly less popular. Papa grew up speaking English and adapted the westernized ways and religion that eventually led to him becoming such a respected person. He recognized that this was the reason for his success, and maybe this caused him to have the thought that old traditions and beliefs are evil permanently dug into his head, which is why he didn't not respect his own fathers ways of living.
I don't get the impression that Papa is proud of his Nigerian background and wishes to be more like the white people he was taught by in the past, and this he forces upon his family.
In conclusion, I don't think Papa is actually a monster, but because of the way he grew up, it influenced and swayed him into a certain direction and state of mind that he probably wouldn't have had if he was brought up like any other Nigerian person living and attending a normal school.
