Why does Mrs. Yeobright die in The Return of the Native by Thomas Hardy?

Among the three victims of Egdon in Hardy's novel,
The Return of the Native, Clym's mother, Mrs.Yeobright, happens to
be the first. On a hot and humid day of August, she walked a long distance to the
cottage of her son to seek a possible reconciliation. Clym, serving as a furze-cutter,
had been living with his newly-married wife, Eustacia.

In
fact, Mrs.Yeobright unknowingly followed her own son to reach the cottage. But Damon
Wildeve chanced to enter the cottage to talk to Eustacia as the tired mother chose to
have some rest. When Mrs.Yeobright knocked at the cottage-door, her son was already fast
asleep, and Eustacia was engaged in a conversation with Wildeve. So neither of them
opened the door, and Mrs.Yeobright had to return, frustrated with the impression that
her son had refused to open the door for the mother.

While
returning all the way from Alderworth to Blooms End, the old exhausted mother was bitten
by an adder. She was taken to a shed nearby and the rustics made efforts to save her by
applying adder-oil. Even a doctor was being sent for. However, Mrs.Yeobright died
broken-hearted, as much because of the adder-bite as being severely bitten by the idea
that she was rejected by her only son, Clym.