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He started seeing them a few weeks before Tara's death. To him, the Doctors were terrifying visions that appeared from the shadows, watching him in his bed, from the treeline while his friends played happily in the park, through the classroom windows making it impossible to focus. After the first time, he tried to tell Tara, who did her best to assure him it was just a nightmare, her eyes flicking to the bedroom door behind him periodically. For both of their sakes, Theo tried his best from that point on to act like things were completely normal.
All day he waited, paranoid, for their next appearance. As terrifying as they were, he knew there was no way they could be real and there was a very real threat that he would react to their presence. It almost terrified him more than the ominous supernatural entities did that he might let someone know that anything was less than totally perfect about him, much less that he might be losing his mind. He was really starting to like Beacon Hills and as much as he knew Tara would say she didn’t blame him for their relocation, he also knew he would hear her trying to muffle her sobs into the pillow at night. Or maybe she would be fine - maybe he’d be the only one sent away this time.
Only Stiles ever noticed Theo staring into space during one of the Doctors’ appearances and Theo only startled a little before laughing and brushing it off with some excuse. Stiles seemed to accept it - even at nine years old Theo was already getting pretty good at wearing the right face at the right time, a habit learned from years in and out of homes with Tara, trying to market himself as worth keeping. But as well as he hid it, the Doctors in their dark cloaks and gas masks weighed heavy on his mind.
By the time the day it happened rolled around, Theo hadn't slept much at all in weeks. He was tired of being afraid all the time and not being able to tell anyone about it. So when the Dread Doctors finally spoke to him, he listened. He thought that if he just listened to what they wanted, they might go away.
One of the Doctors led him through the woods talking about power, about things Theo had never imagined, impossible things, and how some, like the Doctors, lived on the right side of impossible - where they were the feared and not the fearful. The Doctor talked until they reached the bridge, where he saw the other Doctors with Tara standing on the bridge between them, trembling. Her wide open eyes met his with more complete terror than he had ever seen in them. She opened her mouth to say something, maybe to call out to her brother, but it twisted into a scream of agony as one Doctor swung their cane, connecting in a sickening crunch with her ankle. Before she could even finish screaming she was pushed into the water.
Theo was frozen. Part of him hoped, desperately hoped, that he was just imagining it. They didn't really have his sister. It was just another one of their illusions meant to scare him. That wasn't really Tara who he just watched fall into the water.
She surfaced, spluttering and crying, moving despite the pain as far away from the bridge and the doctors as she could, only making it a ways away in the creek before it was impossible for her to traverse in her condition. She looked back at her brother with wild eyes, manic with pain and terror, her gaze not moving from him, and he realised that Tara was only terrified partly for herself.
The doctors kept talking about power. About those who would always live in fear without it because they were too weak and foolish to take it, clinging to sentimentality and other diseases of the psyche to their downfall. Theo listened because he couldn't do anything but listen, stuck staring across the creek at what he desperately hoped wasn't really his sister. By the time they had explained as much as they felt like explaining to the nine year old kid they'd picked as their prodigy, Tara had begun shivering violently. They led Theo to the bridge railing and told him to watch. They told him to leave his weakness behind, leave the version of himself that feared loss, and learn to welcome it. To watch as his sister gave him the chance to live a life on the right side of impossible, where he would be feared - not fearful.
And Theo was still frozen.
The Doctors withdrew to some distance away, leaving their subject to soak in the all-encompassing horror of watching someone he loved dying. They left him to process their words until he understood that he was the reason that Tara was going to die, and it was his fault, and he would live to be better and more powerful while she would not. Tara lifted a weak hand to him as if she could reach him somehow, saying his name. She was pleading with him to move. She was telling him to listen to her voice and come back to him, telling him it was alright, it was going to be okay. She was his big sister and if he got her out of this she'd never let anything happen to him ever again. Theo started shaking, full body tremors overcoming him. Just as he began to think, wildly, desperately, that he might be about to move… one of the Dread Doctors drifted back into his peripheral view. There was no expression to glean from the mask and nothing about their posture betrayed any concern. The Doctor already knew that Theo wouldn't- couldn't do anything. Their appearance was little but a reminder.
The momentary thought that he would be able to move faded back into the roaring wave of terror enveloping his thoughts. Tara kept pleading with him as her lips turned bluer and her cries became more desperate. Theo never moved. He couldn't, not when all it took was one of the Doctors appearing in his line of sight for him to feel like he was drowning in fear and helplessness. But he didn't stop shaking until hours and hours later, when it was over.
In the years that followed, he learned to be a monster. He saw horrors worse than any person could imagine, let alone a kid of his age. The day of his sister's death moved further behind him and he never had a chance to recover from the initial shock of the event. He simply kept existing, his new heart continuously beating. And as the time passed and his memories of what really happened that day were drowned by the memory of his fear and heartache, the Doctors continually reminded him of where his new power came from - by exploiting the weakness of others, and banishing weakness in himself. They said he had earned his heart through what he had done. A new narrative of those events took shape in his mind, one that allowed him to make sense of what happened so that he could become who he needed to become to survive. It didn’t matter that he couldn’t have done anything to save her even if he’d been able to move past the fear. It didn’t matter that fear was the thing that had stopped him in the first place. He came to understand Tara's murder by the Dread Doctors in front of his eyes not as something that happened to him but something he was directly responsible for, something he let happen because of what it gained him.
The Doctors said, years later, that Theo had taught them that true evil came from corrupting something truly good. What they didn’t tell him was that the best way to make someone into a monster was to convince them they already were one.
I killed my sister. I stole her heart.