Pieck doesn't know how she could have ever trusted him. She should have known from the start that this would end with a knife in her back. Still, she had allowed these two brothers to lead her to this cliff, pretending all along that she would not be the one that would be pushed over the edge in the end. She is not entirely to blame. After all, she was not the only one who was fooled. Her comrades, her superiors, and the whole of Marley had been betrayed by putting their faith in these brothers - faith that they would be too scared to act against Marley, faith that their loyalty was to Marley, faith that they also believed that all of Paradis deserved punishment for crimes that had been committed centuries ago. Zeke had earned the country's trust by serving as a soldier for almost thirteen years. This didn't include the time he had spent training to become a Titan shifter as a young boy. He had spent much of his life on the battlefield with Pieck, who had considered him her closest companion. It made sense that she would trust him so much. It didn't make sense at all that she trusted Eren Jaeger.
Unlike Zeke, Eren had never fought alongside Pieck. He did not share the same history or background. He had lived in an entirely different world than her, oblivious to the battle that he and his people were fighting until Paradis had finally escaped the Walls that had been meant to keep them away from the rest of the world. Of course, he couldn't possibly have shared the same goals as Pieck and the other Marley warriors did, so why did she believe him when he said he was joining forces with his brother to bring down the kingdom of Eldia once and for all?
It must have been the look of him, so similar to his brother, that made Pieck trust him so readily. Truthfully, he looked nothing at all like Zeke. When she had seen him amidst the destruction of Shiganshina, she could hardly believe that Eren was related to Zeke. They had nothing in common, and Zeke later admitted to her that Eren looked nothing at all like their father, and yet he continued to insist that there was no mistake about it - Eren was his half-brother. Pieck had only begun to see the resemblance after Eren had snuck away from Paradis and slipped into Liberio, swearing his allegiance to the country and vowing to help them end the existence of Eldians on Paradis. It was the way he looked at her, she thinks, fire in his eyes as he swore his loyalty that made her trust him in the end.
Although Zeke and Eren look nothing at all alike, that intense look in their eyes is something that they both shared. Finding that fire, that passion, so familiar, Pieck had been drawn to it. She should have known better than to become attracted to something so wild and dangerous. Looking back on it, she really had no reason to trust him so much. She shouldn't have allowed him in all of those meetings with the other Marley warriors. She shouldn't have given him permission to listen in on their private plans for the Eldian invasion. She shouldn't have given him information about the Marley military or the extent of the powers of the Titans the country held. Even now, she's not quite sure why she did all of those things. She had treated him like a comrade despite only knowing him for only a few months.
No, she had treated him as more than a comrade. She had treated him like a friend. She had whispered secrets about the city to him, shown him where she and the others had grown up and told him the little crevices that she would hide in to escape from Marley officers as a child. She had offered him food, treated him to traditional Marley cuisines from food stalls and had brought him meals of her own that she had cooked. She had smiled at him and offered him her hand as a friend instead of keeping her distance like she should have.
Had he thought about this when he set fire to Liberio? Had she crossed his mind for even a second as he tore down her city, filled with people she hated and people she loved, and the only home she had ever known? Had he cared that his betrayal would hurt her in the end, or did he go through with it, knowing all the pain she would suffer as he drove the knife further into her back? But even so, she tried to convince herself that he had felt some sort of remorse as he watched the city go up in flames. After all, had he not felt this same despair as he watched his own town destroyed underneath the feet of Titan? But she knows better than to think he would ever feel remorse for his actions. He had not done this because it had been the only thing he could do. He had done this for revenge. He had done this for his people. He had done this for himself, and she is a fool for trying to convince herself that he was ever on her side.
When it comes time to invade Paradis, Pieck willingly goes along with the others. She tells herself that she is doing this to avenge the deaths of the innocent and rescue Gabi and Falco, but she knows this is a lie. In reality, she does this for him. She wants to search for him, wants to see him again, wants to ask him why he had betrayed everyone into believing he was on their side even though she knows he will never speak the words she wants to hear. She knows this, and yet she still holds hope even when she finally tracks him down and holds a gun straight at him, her finger trembling as she threatens to shoot.
"Stop, or I'll shoot," she tells him, but the quiver of her lip betrays her.
"You won't," Eren says calmly as he raises his arms calmly above his head. He does not seem distressed at seeing his former comrade threatening his life or worried that his only hostage Gabi has now run behind Pieck for protection. "Either of us can turn into Titans if we get hurt, but the transformation would kill the girl. You won't shoot." They both know there's another reason why she won't shoot, but it is left unsaid.
Bitterly, she lowers the gun, a look of pure revulsion on her face, but it is unclear who she is more disgusted with - Eren or herself. It's him, she thinks, for being so heartless, for betraying everyone, but she knows that it is really herself that she hates for falling for him anyway.