(Title at the end of the chapter.)
Eren was using Becky's opponents as litmus paper to test her resolve.
If she passes the test and breaks the siege, he would help her get rid of her pursuers. After all, he had invested a lot in the mage. She was more useful to him alive than dead.
Becky might be able to transform herself after suffering through such a life-altering ordeal. She might be able to fix her attitude, allowing her to become a capable member of Eren's team. The butcher wouldn't have to deal with her as a nuisance when that happened.
But if she fails and gets killed in the test, he could make use of her in a different way. Either way, he could gain something from this fight.
At this point, Becky had come to understand that nobody was coming to save her. And the aggression she had found within her had no use anymore. Scared and lacking hope, the mage started to flee the battlefield.