The motivation for killing Mademoiselle Source was mainly for her inheritance and assets and he was simply evil.
Step-by-step explanation:
Guy de Maupassant breaks the stereotype of a personal character of an orphan in the story "Orphan".
The young man in this story is raised by Madam Source and always provided him everything he wanted. He too lived a reserved and facade life in front of her and also an ungrateful murderer who becomes a mayor later.
The motivation for killing Mademoiselle Source was mainly for her inheritance and assets and he was simply evil. Till the very end, no one really knows who he truly is and what he is capable of.