Eugene Hamel wanted to become the Marquess to be more powerful than his older brother.
The investigation revealed that the Hamel family's issues led to the siblings not getting along well.
Eugene particularly had a bad relationship with his elder brother, the firstborn.
Because Eugene was born to the second wife, he had insecurities when it came to the standing of his elder brother. Especially since the first wife always rubbed her influence in his and his mother's face.
The treatment between them was starkly different, too, since Eugene's elder brother was educated and trained as the heir, so he had expensive and expert tutors. Meanwhile, Eugene's teachers were different.
Thus, Eugene wanted to prove that he could be so much better than his older brother by becoming the Marquess.
Of course, Eugene also had twisted tendencies and greed for power aside from his envy of his older brother.
