Arsenic poisoning has symptoms which are consistent with the symptoms reported about Russell's condition. A person can be slowly poisoned to death with arsenic, which would have made it less obvious to suspect as cause of death in Russell's day.
Rutherford had the most to gain by Russell's death.
Russell's body was hastily buried without any autopsy, as I recall.
Rutherford was a ruthless, self-serving asshole who had no morals and no shame.
This is all circumstantial evidence, except for the last item.
Farkel