Bakura's body was not his own.
This much should have been obvious the moment he mentioned his "host"—the poor innocent boy Ryou whose strange pendant housed a 3000 year old spirit. Bakura and Ryou were entirely different people, however much Bakura seemed to dismiss this by taking Ryou's surname as his own. Most people had simply given up on trying to distinguish who was in control of the body at any given point, and simply resorted to calling this peculiar entity "Bakura."
Marik could tell the difference. All it took was a glance at his posture, the way he held his head, and Marik knew. The look in Bakura's eyes—but they weren't his eyes. They were stolen, every bit of him was stolen. Marik knew this; yet the more time he spent with the boy, the less he found he could accept it. Ryou was soft-spoken, caring, and sincere; the sinister spirit within him was anything but. Ryou would bow his head and avoid others, Bakura would hold his chin high and force people to acknowledge him. Such frighteningly different souls, and yet they were the same.
Marik refused to acknowledge the similarity. And he had witnessed the shift himself. One minute he was standing on the docks with that sinister ancient spirit, completely enthralled as he watched him carve their plans—their partnership—in his arm; the next, the boy was waking up beside him an entirely different person. Marik had been with the body the whole time, had seen it inhabited by two different minds in one sitting, and still he refused to acknowledge it.
It was the eyes, Marik decided, that caused him so much trouble. Those narrow eyes that bore his cynical laughter, that could stop Marik's words dead in his tracks—the windows to the soul, he had read once. Were they also the window to Ryou's soul, then? Impossible. Ryou could never bear that hideously terrifying glance, his eyes could never sparkle with twisted delight at the defeat of his foes. A quick glance of acknowledgement from Ryou could never make Marik's chest tighten so hard that his heart was forced into his throat.
Those eyes belonged to Bakura as much as anyone.