In "The Adventure of the Speckled Band," what information does Holmes gather to start his investigation after Roylott's visit?

Holmes learns that Roylott is very strong and also unstable.

A woman named Helen Stoner visits Holmes to tell him that she fears that her sister’s death may not have been an accident. She thinks her stepfather, Dr. Roylott, was responsible. Stoner tells Holmes that her stepfather is violent and quarrels with the neighbors.

After Stoner leaves, Roylott storms in, and from Watson’s description he certainly seems dangerous.

So tall was he that his hat...

Holmes learns that Roylott is very strong and also unstable.

A woman named Helen Stoner visits Holmes to tell him that she fears that her sister’s death may not have been an accident. She thinks her stepfather, Dr. Roylott, was responsible. Stoner tells Holmes that her stepfather is violent and quarrels with the neighbors.

After Stoner leaves, Roylott storms in, and from Watson’s description he certainly seems dangerous.

So tall was he that his hat actually brushed the cross bar of the doorway, and his breadth seemed to span it across from side to side. A large face, seared with a thousand wrinkles, burned yellow with the sun, and marked with every evil passion, was turned from one to the other of us…

Roylott is indeed angry. He found out that Helen Stoner has been to see Holmes and he wants to know why. He calls Holmes a “busybody” and a “meddler.” He also threatens Holmes, telling him he is a “dangerous man to fall foul of.”  When he bends a fireplace poker with his bare hands, Holmes learns that he is not only unstable but incredibly strong.

Holmes is not impressed. He unbends the fireplace poker just as easily as Roylott bent it. Holmes now knows what kind of man Roylott is.

This incident gives zest to our investigation, however, and I only trust that our little friend will not suffer from her imprudence in allowing this brute to trace her.

Holmes is not the least bit intimidated by Roylott. If anything, he is amused. Watson and Holmes go to Stoke Moran to learn how Roylott killed Julia and planned to kill Helen. Waiting in the dark, Holmes realizes that he has sent his trained pet swamp adder out as a murder weapon. Unfortunately for Roylott, the murder snake turns on him and he is killed, saving everyone the cost of a trial!