Eren had already noticed that Leo and Anya weren't friendly with each other. Although they weren't extremely hostile to each other openly, their internal equations had long become clear for Eren thanks to the options.
If the options wanted him to use one of them to kill the other, it meant that there was a high possibility that they did want to kill each other in the first place. They just needed a little push.
Eren didn't want to make them kill each other since that was only going to create more complications for him. However, he could definitely use that enmity as a form of shield.
It was also why he didn't directly attack Anya, waiting for Leo to interfere. His plan had almost failed when the Undead Dragon came for him, but in the end he had succeeded. His bet had paid off.
"Leo, what do you think you are doing?! Do you really think I am afraid of you?" Anya's rageful aura rose straight to the sky.
