Harold Shipman committed his crimes when euthanasia was illegal, which illustrates that psychopaths can commit crimes whatever the legal situation. Legalising euthanasia would have no effect on the 0.000001% of society who do this sort of thing.
In countries where euthanasia is currently legal, such as Switzerland and the Netherlands, strict legal guidelines are in place to ensure that the process does not include such problems. All patients who request euthanasia require the diagnoses of at least two doctors to verify the terminal nature of their illness, and undergo psychological examination by these doctors and often other experts to examine the reasons for their choice. It is not a situation of "Surely it is wrong to give one or two individuals the right to decide whether a patient should live or die?"; it is one of two medical professionals deciding whether the legal parameters allow them to enact the patient’s wishes. [[ Dutch Ministry of Foreign Affairs"A Guide to the Dutch Termination of life on Request and Assisted Suicide (review procedures) Act – April 2002" p3 Accessed on 01.06.09]]
It is worth noting that, at the moment, doctors can effectively use euthanasia anyway. Firstly, under the "doctrine of double effect", a doctor is allowed to give a patient, upon their request, a dose of painkilling medication which as a secondary effect speeds up the death of the patient. [[ Alison McIntyre "Doctrine of Double Effect" Stanford Encyclopaedia of Philosophy Accessed 01.06.09 ]]Secondly, all patients have both the right to refuse treatment, and the ability to make a "living will", which doctors are compelled to consider if the patient is unable to express their wishes during illness. [[ Direct gov "Government, citizens and rights- How to make a living will-http://www.direct.gov.uk/en/Governmentcitizensandrights/Death/Preparation/DG_10029429 01.06.09]]