The author described The Black Hearts as the darkest fic in their series, and they were not wrong. Mercury was truly "born in a nightmare."
- Marcus Black is a truly awful parent. And he’s the source of most of the nightmare fuel in this story. Not that much is known about him in canon, so the story takes the time to fill in the blanks. And it doesn't look pretty.
- He was a violent psychopath long before he ever became an assassin. He worked in the Atlas military just to kill people and was discharged after an assault on Vacuo. He basically picked his profession to commit government-sanctioned murder. His profession as an assassin is to basically so he can get paid to be a serial killer.
- At the beginning of the story, Mercury says that Marcus has at least seventy kills to his name. There's are a few Time Skips in the story, so by the time he is killed, the number has likely risen significantly since Mercury's initial remark.note And he hadn't even been arrested for any of them.
- His abuse of both Mercury and Melanie is terrible. He goes on a hit and drags Mercury along. When Melanie just wants them to stay for dinner, he slams her face into the fridge, leaving blood on it. He even beats her into submission for retaliating. And he will beat both of them for some rather minor slights as shown in Chapter three. In the bonus chapter, Marcus just randomly breaks Melanie's wrist while laughing about it.
- One thing abusers will do is to try and justify the abuse they commit. Mercury assumes this is the case with Marcus' training sessions. He basically uses them to beat up his son under the guise of helping him fight. While it isn't outright confirmed that he is intentionally doing this, his sadistic personality still lets him enjoy it all the same.
- During this hit, he holds the target’s daughter hostage while he forces Mercury to shoot him in front of her. Despite Mercury’s protests, Marcus insists on him killing the father first. Marcus later punishes him for hesitating in committing this act of cruelty.
- He was a violent psychopath long before he ever became an assassin. He worked in the Atlas military just to kill people and was discharged after an assault on Vacuo. He basically picked his profession to commit government-sanctioned murder. His profession as an assassin is to basically so he can get paid to be a serial killer.
- Melanie doesn’t slack off in this respect either. She straight-up threatens Marcus at gunpoint just to get him to come upstairs for dinner. She even threatens to kill his target who she knows he’s getting paid extra to torture. It shows another disturbing thing about Melanie. Even though she knows about her husband, she doesn’t even care about his victims in the slightest.
- There's also the victim's perspective in all of this. She's being taken by a crazed assassin preparing to torture her and his son only stands by and helps. Melanie comes downstairs to stop them, but only so they can come up an eat dinner. Nothing more than a Hope Spot for her and she has to stay in that basement waiting for Marcus to finally come back down and get to business.
- The bonus chapter provides another piece of disturbing content. Melanie is such a vicious homophobe that she'll demand that Marcus kill him right on the spot. If she didn't get her wrist broken by Marcus, she might've done it herself.
- And just to be sure everyone is clear, all of the above has basically become routine for Mercury. He had to live with those two sociopaths and put up with the crap they committed for over sixteen years. And he spent a year of his life alone with the worse one of the two. As Mercury notes, this was very damaging to his psyche. It's not surprising that Mercury ended up the way he is in the show. In fact, it's surprising that he didn't turn out even more messed-up than he did.
- Jaylin Michaels is an even worse homophobe. He hates Cobalt so much that he would kill his wife just for accepting Cobalt's sexuality. And Marcus is the one to drop the news to him. Imagine how spiteful Jaylin had to be to set Marcus on Azura just for defending his son's sexuality...
- The ending fight with Mercury and Marcus has its fair share of scary stuff. For one, it shows how truly Ax-Crazy the latter is.
- There’s the moment where Marcus is viciously beating his son on the table while attempting to kill him.
- When Mercury thinks he left Marcus to burn in the house, his father comes out behind him and attacks him with a pipe out of nowhere. And he comes back in full force.
- The way that Marcus comes straight for Mercury to kill him while Mercury's kneecaps are blown out and he his helpless to stop it is surly menacing.
- Marcus's death, while brief, is described by him choking on his own blood after Mercury shoots him in the neck. He definitely deserved it, but it was quite a cruel end.
- In the end, Mercury is left without a home, without legs, and fatally bleeding out. Even he noted he wouldn't make it far. Had it not been for Cinder and Emerald showing up, it wouldn't have been long before he died without any hope of rescue.
