After drinking a potion of his own creation, Jekyll is transformed into the smaller, younger, cruel, remorseless, evil Edward Hyde, representing the hidden side of Dr Jekyll's nature brought to the fore. Dr Jekyll has many friends and has a friendly personality, but as Mr. Hyde, he becomes mysterious, violent, sadistic, and secretive. As time goes by, Mr. Hyde grows in power. After taking the potion repetitively, he no longer relies upon it to unleash his inner demon i.e. his alter ego. Eventually, Mr. Hyde grows so strong that Dr. Jekyll becomes reliant on the potion to remain himself.
Stevenson never says exactly what Hyde takes pleasure in on his nightly forays, saying generally that it is something of an evil and lustful nature; thus it is in the context of the times, abhorrent to Victorian religious morality. Hyde may have simply been reveling in activities that were not appropriate to a man of Jekyll's stature, such as engaging with prostitutes or burglary, or perhaps merely walking the dark streets of London allnight and sleeping during the day. However, it is Hyde's violent activities that seem to give him the most thrill, driving him to attack and murder Sir Danvers Carew without reason, making him a hunted outlaw throughout England.
Realizing he will soon be Hyde forever, Jekyll leaves behind a testament; pointing out that while Jekyll often felt like a charlatan, Hyde felt like a "genuine man" years younger and far more energetic than his more "sociable" self. He also states in his final confession that although Hyde knew people recoiled from him, he did not care.