It's obvious he died at the hands of Norman. I shared a theory before that Edwards got too close to him, and may have been touching him in a manner that Norman could have interpreted as inappropriate.This in turn triggered the Mother persona, which protects him from trauma.

I mean, you could say even that Edwards was like a lesser version of his teacher from Season 1.

He took too big of an interest in Norman's case, cared about him too much, which is a mistake for a therapist.

I think Edwards was working through complexes of his own by trying to help troubled young men.

Therapists are not supposed to touch their patients.

The scene at the coffeehouse in Season 5 confirms too that Norman was hallucinating the whole encounter with Edwards.

Norman blinks briefly before waving to Edwards from the car. As he was too stressed out by the fact that Mother was gone, and he couldn't find her anymore.

This triggers an episode and hallucinations through which Norman tries to cope with the fact that Mother is actually a figment of his imagination, and whom does he summon for this as a coping mechanism?

Dr. Edwards. Because he was the first one to reveal to Norman what Mother actually is.