Moreover (according to a five minute wikipedia search), not all doctors swear the same oath, but the modern version of the Hippocratic oath does not have an explicit "Thou shalt not kill" provision and in fact, it doesn't even include the commonly quoted "First, cause no harm".

Obviously taking a person life, even with his/her consent, may violate the personal ethics of some people, but if that is the problem the obvious solution is to find a different doctor.