Friendship - even though he had a falling out with Jekyll, when Jekyll's letter arrived he still felt bound to help him, his friendship ultimately causing his death
Quotes:
"Were by no means in the habit of correspondence"
"You are one of my oldest friends"
"Although we may have differed at times on scientific questions"
"I cannot remember, at least on my side, any break in our affection"
"‘Jekyll, my life, my honour, my reason, depend upon you,’ I would not have sacrificed my left hand to help you."
"If you fail me to-night I am lost"
"This drawer I beg of you to carry back with you to Cavendish Square exactly as it stands"
"my heart sinks and my hand trembles at the bare thought of such a possibility."
"I felt bound to do as he requested"
"an appeal so worded could not be set aside without a grave responsibility"