Why is Dr. Watson angry with Thaddeus Sholto in The Sign of Four?

The answer to this question lies in chapter 4.
In that chapter, Miss Morstan, Holmes, and Dr. Watson meet Thaddeus Sholto. Thaddeus is about thirty years old and is the son of Major Sholto.
We learn that during the course of the visit, Thaddeus crudely reveals that Miss Morstan's father died from a heart attack. Thaddeus Sholto's unfeeling behavior angers Dr. Watson.
In fact, Dr. Watson is so angry with Thaddeus that he admits being tempted to strike the man across the face.
As the chapter continues, we learn that Thaddeus and his twin brother, Bartholomew, are the twin sons of the deceased Major Sholto.
Before Major Sholto's death, he revealed to his sons that he and Captain Morstan were in possession of the stunning Agra treasure. When the latter came to claim his portion, however, he ended up in an argument with Major Sholto.
Captain Morstan's anger proved fatal: he suffered a heart attack and died.
At the end of the chapter, we learn that it was Bartholomew who found the treasure's hiding place in Major Sholto's former home. Apparently, the Agra treasure, consisting of jewels, is valued at not less than half a million sterling.