The supernatural elements of the plot, including Jekyll’s secret potion and ability to split his consciousness in two, allow for a kind of situational irony to arise in the last chapters of Dr. Jekyll and Mr. Hyde. That Henry Jekyll—a pillar of Victorian society in London—is the murderer of a member of Parliament and a sadistic criminal is an irony in itself. But a more specific example of situational irony is that Hyde’s pull over Jekyll is rooted in the very last place anyone thought to look: within Jekyll himself.
Consider Lanyon’s surprise when he learns the truth, as reported to Utterson:
“ I have had a shock," he said, "and I shall never recover. It is a question of weeks. Well, life has been pleasant. […] I sometimes think if we knew all, we should be more glad to get away."
The intensity of Lanyon’s shock (“I shall never recover”) speaks to how deeply unexpected this turn of events is. The knowledge of Hyde’s true identity has not only estranged Lanyon from Jekyll, it has also given him a distaste for life itself (“if we knew all, we should be more glad to get away”). The event has totally reorganized Lanyon’s view of the world and what people are capable of.
Utterson, Poole, and others in the novella put forward various theories about the relationship between Jekyll and Hyde, all of them more or less plausible. That Hyde is an implicit part of Jekyll’s personality is beyond the imagination of even the most cynical of Jekyll’s friends. This inability to reconcile the two figures as being one takes on quite a bit of significance, as the characters in the book are unable to reconcile the worst and best traits of a friend they thought they knew.