Why did the Summoner kill Mindfang? I thought THEY were on the same side

The details of Mindfang’s death were never revealed in-comic so all we know is that he killed her. In the fan animation After Us by toastyhat Mindfang controls the Summoner to kill her to put her out of her misery when she was injured and dying. That’s one way to explain her death. We just don’t have enough information in-canon.