Mark is Jack's abusive, alcoholic father, with whom Jack had a love-hate relationship. Although he's dead, Mark is very much alive in the mind of his son, who's deeply afraid of repeating all his father's mistakes. Making Jack's dad a male nurse, who frequently comes home bloody (some of the blood probably from fighting) is a creepy gruesome touch. Under the influence of the Overlook, all of Jack's issues with his father become manifest and result in Jack's own destruction. Of course, it also results in the destruction of the Overlook, a seat of extreme evil, especially for families.