@MarqFJA87
He doesn't get angry because he's just a doting father. It's the Dou effect.
Chapter one says that the reason he halted his conquest was because Dou was too kind and it worries him.
Jahy is only acting upon that premise.
No matter what the father says about him being okay about it, he's still not doing any conquering.
What Jahy resents isn't Dou's kindness and ways, but Dou herself.
If the demon king resumes world conquest, I think Jahy likely wouldn't care about Dou's kindness or being not traditional.