Is not that so ?" He seemed to bite both men with the question.
"Yes," they answered.
"Then he was compelled to hang about the avenue, watching his opportunity--his opportunity for what?
Not to commit a murder! He was unarmed, or, at any rate, his implement was a haphazard choice, selected on the spur of the moment.

He saw David Hume leave the dance, and watched his brief talk with the butler.

He correctly interpreted Hume's preparations to await his cousin's arrival.

Did Hume's sleepiness suggest the crime, and its probable explanation?
Perhaps.

I cannot determine that point now.