Thank you for your question.
If I may be permitted to make an observation, it is not always helpful to isolate a few words and expect explanation. Context - I feel - is essential to the understanding of meaning. The quote you have identified comes from Jekyll’s full statement. It centres on the emotions and feelings that Henry Jekyll had after returning from one of his excursions as Edward Hyde. The full paragraph is below.
“When I would come back from these excursions, I was often plunged into a kind of wonder at my vicarious depravity. This familiar that I called out of my own soul, and sent forth alone to do his good pleasure, was a being inherently malign and villainous; his every act and thought centred on self; drinking pleasure with bestial avidity from any degree of torture to another; relentless like a man of stone. Henry Jekyll stood at times aghast before the acts of Edward Hyde; but the situation was apart from ordinary laws, and insidiously relaxed the grasp of conscience. It was Hyde, after all, and Hyde alone, that was guilty. Jekyll was no worse; he woke again to his good qualities seemingly unimpaired; he would even make haste, where it was possible, to undo the evil done by Hyde. And thus his conscience slumbered. ”
At the heart of this novella is an understanding of what Robert Louis Stevenson meant by “duality.” The simplistic explanation is that we are talking about two different individuals who in habit their own body mass. I would argue that is not what Stevenson meant. He is referring to a single individual who had the ability to change both his personality and certain physical characteristics.
So to put it simply Henry Jekyll is Edward Hyde and Edward Hyde is Henry Jekyll. Ot to put it even more simply there only is Henry Jekyll. Whenever we read about Edward Hyde we are really seeing Henry Jekyll transformed by a medical potion.
The problem is that the medical potion has the ability to change certain physical aspects of the anatomy. This allows others to believe there are seeing another person - when infant what they are seeing is Henry Jekyll who has undergone certain physical changes as a result of taking the potion.
At the heart of this duality problem is Henry Jekyll’s ability to separate these personalities and create the illusion that Edward Hyde is not Henry Jekyll. Henry Jekyll appears able to believe that when he is under the powers of the potion he is not Henry Jekyll - he is a different person called Edward Hyde. But the truth is he is Henry Jekyll at all times.
I apologise if I have spent a long time on this issue, but in my view it is a point needing to be understood to be able to properly interpret this passage and the novella.
Now for a detailed explanation of your quotation.
At the beginning of the passage Henry Jekyll comments “When I would come back from these excursions, I was often plunged into a kind of wonder at my vicarious depravity.” Henry Jekyll is a pillar of London society. However the irony is that he is a flawed character who - and this is a complete surprise to him - deep inside him enjoys the sexual and depraved behaviour that is only made visible when in the character of Edward Hyde.
He is utterly shocked by what he - Henry Jekyll - was like and behaved like when he was Edward Hyde. As he says he saw a man who was “being inherently malign and villainous; his every act and thought centred on self; drinking pleasure with bestial avidity from any degree of torture to another; relentless like a man of stone.” Henry Jekyll’s language is really interesting. He says he “saw a man.” He is not prepared either here - or elsewhere in the novella - to take ownership of who he really is - especially when under the powers of the potion. Henry Jekyll maintains it is not he who committed these actins - it is/was someone else. Throughout the novella Henry Jekyll is in complete denial of this point that Edward Hyde is actually Henry Jekyll. And what Edward Hyde does has actually been carried out by Henry Jekyll.
To pick up your actual quote Henry Jekyll enjoys his behaviour when out on an excursion as Edward Hyde. However he is also shocked when he remembers Hyde’s “drinking pleasure with bestial avidity from any degree of torture to another; relentless like a man of stone.” As Henry Jekyll he would never involve himself in the kinds of drinking that he indulged in as Edward Hyde. There is an irony here. The Victorian morality that Henry Jekyll personifies in London public is just a mirage. The real Henry Jekyll is described by the behaviour of Edward Hyde. It comes as a real shock to Henry Jekyll to discover that deep down he is that kind of a man.
One of the themes that Fanny Stevenson - his wife - was unhappy with was the pronounced description of homosexuality in the first draft of the novella. Partly because there was a heated argument between Robert and his wife that resulted in Stevenson throwing his first draft into the fire and burning it and patly because he took on his wife’s criticism the issue of homosexuality was removed from the final draft. The said, the reference to “bestial avidity” is one of the few references about the issue of homosexuality that still remain in the novella.
At the end of the passage is the most serious criticism of Henry Jekyll. His total disregard to taking ownership of his actions colours him far worse than any criticisms of Edward Hyde. In a completely cynical reflection Henry Jekyll says “It was Hyde, after all, and Hyde alone, that was guilty. Jekyll was no worse; he woke again to his good qualities seemingly unimpaired; he would even make haste, where it was possible, to undo the evil done by Hyde. And thus his conscience slumbered.” Because he is able to change into and out of Edward Hyde, Henry Jekyll refuses to take any responsibility for his actions.