If I remember correctly and if this reason is correct, the white haired guide killed the guide of the brown haired esper. This esper though is an S Class, and maybe he has no other higher match except for the white haired guide. You know how important a guide is for an esper, right? Maybe he cant take the risk of losing a guide for the fear of going berserk without guiding or not finding another guide after that.
I think it’s more than that. This guide is an esper made guide. I think he has more than just “guide”- abilities and that the esper can’t choose to be with him. He is forced.
It’s all very mysterious.
What hold does he have on that poor esper? And can’t he just burst into flames and die?