Active euthanasia is legal in a few countries for terminally ill patients.
That doesn't seem an accurate description of the situation. Yes, doctors and nurses sometimes 'help people along' in their final hours or even days, that is not the same thing as the euthanasia being described here.
There's no reason it couldn't be a closed system, where any fertilizer that doesn't become part of the crop biomass is recycled. In theory it should be more sustainable than existing agriculture and use less fertilizer per kg of crop produced.