I would want to argue that the most important piece of evidence in this story that convincts Dr. Jekyll is his own testimony that is discovered and relayed to us towards the end. In it, we have Dr. Jekyll's own account of what happened and how Mr. Hyde, far from being some person or being exterior to himself, is very much a central part of his identity and whom is unleashed by Dr. Jekyll's own actions. Note for example the way in which Dr. Jekyll refers to what made him take the potion once again after having vowed never to take it:
I began to be tortured with throes and longings, as of Hyde struggling after freedom; and at last, in an hour of mortal weakness, I once again compounded and swallowed the transforming draught.
Dr. Jekyll goes on to relate how this action sent out "his devil" in all of its passion and anger. Dr. Jekyll is fully aware of the consquences of his actions, and he curiously states that he took the potion in "an hour of mortal weakness," clearly showing his own failings and responsibility in unleashing Mr. Hyde. Dr. Jekyll therefore has to be held accountable, to a certain extent, to the crimes of his alter ego.
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