This fish kill can be summarized as follows:
1) Excess aquatic plants peripherally and heavy plankton bloom in the water column.
2) Someone treated the vegetation, it started to die and decompose, dramatically increasing the BOD (Oxygen demand).
3) The plankton bloom crashed (died) at the same time as the plants were decomposing. The plankton bloom crash was either a result of copper sulfate treatment or it ran its course and was finished off by the heavy rain.
4) The resulting oxygen demand suffocated your fish. You can tell the fish suffocated because many of them have their mouths wide open, jaws locked and gills flared open.
5) The copper sulfate did not directly cause the fish kill. Decaying vegetation caused it. The copper sulfate was the stimulus. It did what it is supposed to do...kill the algae and plankton.
6) There's good news here, though. Your fish kill was probably partial and the pond will recover.

Teach a man to grow fish...
He can teach to catch fish...