Peter Singer has consistently argued for euthanasia based on utilitarian criteria.  Here is an extract from Practical Ethics which concludes:

All of this is not to deny that departing from the traditional sanctity-of-life ethic carries with it a very small but nevertheless finite risk of unwanted consequences. Against this risk we must balance the tangible harm to which the traditional ethic gives rise – harm to those whose misery is needlessly prolonged. We must also ask if the widespread acceptance of abortion and passive euthanasia has not already revealed flaws in the traditional ethic that make it a weak defence against those who lack respect for individual lives. A sounder, if less clear-cut, ethic may in the long run provide a firmer ground for resisting unjustifiable killing.