This quotation comes from the tenth chapter of Robert Louis Stevenson's The Strange Case of Dr. Jekyll and Mr. Hyde, and it is part of a letter written by Jekyll to Utterson explaining his views about the horrific situation into which he has gotten himself. Jekyll realizes that Hyde is inhuman in his cruelty. He seems to be both hellish and inorganic, somehow lacking in real life even in the midst of his energy. Let's reflect a bit more on this.
Jekyll learns the hard way what it means to be truly human when he uses a serum to change himself into the horrible Mr. Hyde, a murdering, vicious creature who hates everyone and exhibits all the darkness of fallen human nature. Jekyll has always felt these dark impulses in himself, and he wonders what it would be like to give in to them just for a while. He doesn't want to do it in his own persona, for he still cares about the respect of those around him, so he develops the serum and releases Hyde into the world.
Jekyll, however, finds Hyde impossible to truly control, and his transformations begin to happen without his permission. He realizes that the darkness has taken over. He has given his impulses an inch, and they took a mile, so to speak. Now, he is trapped by his own darkness, and he cannot see any way out but death. Hyde, Jekyll writes, has no true life beyond the physical. He is totally corrupt, and he is draining Jekyll's life as well. In spirit, Hyde is dead, hellish, rotting, and shapeless.
This tells us something important about human nature. We all have a dark side to our characters, but we must not give in to it. We must strive to resist and control it rather than giving it any freedom. There is a Hyde in every person, but that person has the responsibility to chain up Hyde before he manifests to the world and wrecks havoc that quickly gets out of control.