Can a healer kill a Hitler

situation:
A mass murderer that plans to kill further comes to a doctor.(house season 6;the tyrant)
a)should the doctor treat him?
b)The doctor concedes to treat but plans to kill the patient by intentionally committing the mistake: is this right?

The person on getting healthier is going to kill a big number of people and inflicting a great pain on humanity.Although it might be against the law or risk the doctors life.
The other opinion is this that doctor should do his job and leave the job of dealing with murderer to the people it belongs to.
My opinion is that the doctor should refuse to take this case outright and if he concedes to treat, he should treat honestly. Although it would be best to kill the psychopath but its not the duty of a doctor.