Some people with a pathological interest in the power of life and death can be attracted to medical or related professions. Killers who occupy the role of a professional career are sometimes referred to as "angels of death" or angels of mercy. In this role they may kill their patients for money, for a sense of sadistic pleasure, for a belief that they are "easing" the patient's pain, or simply "because they can". In some cases the killer claims the motive is euthanasia when it is not, the difference being that a serial killer lacks a sense of compassion towards the patient which is expected in situations of euthanasia. Most murders committed by nurses are performed by lethal injection. The typical medical professional who murders kills two patients each month. A 2011 study of characterizing 70 female serial killers found that 30% of the offenders were nurses.