Man, this is a complex topic indeed. While i think euthanasia is 'correct', from an utilitarian standpoint, minimising suffering and what not, I think that the possible disadvantages could outweigh the advantages. Personally, I think we have to consider it from other people's perspectives. Yes, it is painful for the person undergoing the suffering, but how about his/her's family? If the victim is 'killed', wouldn't it affect the emotional wellbeing of their families too? You have to consider that they, as humans, have hope - the hope that he/she can be cured eventually.
Moreover, consider the feelings of the medical staff. How would the medical staff feel upon essentially 'killing' somebody? In addition, You have to consider, once euthanasia is legalised, the mindset that suicide is acceptable would be transmitted through society. People would be more inclined towards assisted suicide. How should the doctors handle this then? Kill everybody? I don't know, but this could potentially cause problems in society, what with mindset issues and the like.