In the back of my mind I am thinking the pilot had a sudden heart problem, or a stroke, rendering control of the aircraft impossible.
I wondered that too. Do not know if it would be possible to determine based on the condition the remains must be in.
If you are not a pilot you don’t know what spatial disorientation is. This is a clear case of it. He had no experience in actual IFR conditions. It is scary if you don’t know how to maneuver in these conditions.