1. Utilitarianism requires people to achieve the most happiness in the world. We can imagine if the law allows the surgeon to do so, people will be afraid of being killed if they can be sacrificed. If we do not ask for the willingness of the healthy people to do so and we force them to sacrifice themselves for other people, the world will be a mess. people do not have a choice to live for themselves and do not know when they will die. All of this will cause a lot of disasters. This does not correspond to the Utilitarianism.
2. Utilitarians may argue that killing the patient would be wrong that if people may feel terrible and society will be likely to collapse. To stop that bad thing happen, Utilitarians can conclude that killing healthy people may reduce the total happiness can make more worries in society.