Do you remember when I said in my Headcanons monty turned his mom to diamond by accident
He keeps her statue somewhere safe ( was his hotel ) and cleans it everyday
I'll explain more in detail below,
Ever since he was a kid, he couldn't help stealing whatever he thought was worth keeping. It started off with small things, to the point even irrelevant things too ended up in his hands.
As the kleptomania developed further, it'd get him even more in trouble with his father who already disapproved of his entire behavior.
His mother had opened him the doors towards art ; music, dance, and literature, and his main ideal was Romeo and Juliet, hence the name. And while he was an art enthusiast, it still wasn't enough to satisfy what his father needed of him. His mother would cover for him, much to his father's dismay, to the point he decided to disown his own son.
Sick of his father's abuse and mistreatment, he didn't hesitate to leave as soon as he was told.
Several years later, he chose to return home ;
After getting his hand on the diamond amulet, he thought he might show it to his mother as an achievement. But that wasn't the only aim of his trip ; the one he'd seen first was his father ー sneaking through the kitchen window, he came face to face with him ; quickly grabbing onto a knife, he stabs his father to death.
Now covered in blood, he realizes his mother had been watching. Knowing she already had heart problems, he watches her fall to the ground. He reaches to hold her, and unaware of the full extent of the amulet's power, he turns his mother to diamond. He wasn't sure if her turning this cold was because she was already dead when he held her, or because she had turned to crystal.
But the fact he hadn't much reacted to her death made him think, that perhaps his own heart had stopped too, turning cold, yet shiny and sturdy like diamond, the first part of him to crystallize.
He realized that day that all his ideals relating to people were going to be disappointing anyway ; valuables such as money or jewelry would never really lose value and power ー and if lost, they could always be found again.
Hence, his mentality shifted to mostly caring for this side of him, thus neglecting his artistic side, burying it along with his mother.
He still has her statue intact. Sometimes he still wonders if he had gone to his mother first, and then to his father ; perhaps things would've been entirely different.
The reason he went to see his father first was because he had a specific goal in mind, to take revenge on him, not only for himself, but also for his uncle.
As a child, he had witnessed his father kill his own brother in order to inherit his wine producing company and be its new boss.
Montague's uncle used to be one of the very few people he looked up to aside from his mother, and it was him who had told him about the diamond amulet, as a farfetched fantasy, keeping him dreaming about it over and over.
All these thoughts were intermingled in his mind, causing him to choose revenge before sentiment, making him the legal new owner of the company. However, turning his mother to diamond was an unwanted side effect, leaving him confused about what to do next ー until he thought the loss of both parents would be too suspicious, especially that he had just returned.
He needed to get rid of the evidence as soon as possible.
[ tw for slight gore and accidental cannibalism ]
He decides to chop up his father's body into reasonably sized parts and turn them to diamond, changing their shapes in order to make them appealing enough.
The only thing he'd been too curious about was his heart. It felt like his true act of vengeance, and the first compulsive thought that crossed his mind was to eat it.
Distraught by his own actions afterwards, it didn't take too long for him to regret, and then continued with his initial plan of burying the evidence and having it spread around. Those cannibalistic tendencies will only show in random impulsive bits, especially after committing a murder.
His only witness had been his mother, just as he had been the only witness of his uncle's murder ー and the two of them will be quiet about them.