I'll share my opinions here both as a reader and as the scanslator. Ps: being part of the scanslation actually helps in noticing a ton of details.
First, we can see that Ardenheid was reading a book, and when his father called for him (presumably to ask what he's doing), he said he's trying to imitate humans, evidently since most demons don't read and are dumb and illiterate. However, from my point of view, it seemed like Ardenheid was genuinely trying to learn, but at that moment of time was afraid of disobeying his father.
His father, being the arrogant man he was, said that there's nothing to learn, and humans will always be weak, so they don't have to grow any stronger since they already have special powers.
However, Ardenheid didn't care, he just wanted someone to fight against, someone who will make him bleed, tear his arms off, even if the enemy was another demon. He was scared of dying without going out with a bang.
In the next scene, Ardenheid kills all his father's puppets and even his lovers. It is safe to assume he started killing the puppets first and later challenged his own family.
Notice that Ardenheid here was significantly bigger, meaning that time has passed and he has gotten stronger. With that, he has concluded he was now strong enough to take over.
When confronted by his father, he deems him (the father) unworthy to be a ruler, since he neither improves himself, nor does he try to help his race advance and develop.
"Life is like a stage, waiting for the curtains to close. We must take action for ourselves for something to change. Dear father, please leave the stage"