The cause of Lucas' death was the madam's anger upon discovering Aria's existence. She saw a glimpse of the ecstatic Lucas as he hid around the manor just to procure knowledge from this maid.
Both of them were an eyesore to her, who swore that she would never let Lucas live on happily.
The reason for her actions was not just because Lucas caused her pain and troubles. There was a much larger reason for it. It was not because of the prophecy that came with Lucas' birth. That was just a part of it, but there was something that scared her much more than the prophecy did.
There was a man that came to her. A man wearing a hood that warned her of Lucas.
He didn't say that Lucas will bring her bad luck. He specifically said that in two decades, Lucas Delmonte will end their lives and ruin the Delmonte name as a whole.
The man's identity was unclear, but before he left, he stated some pivotal information that made the madam believe him more and more.
