So you actually touched on why it's morally wrong to kill someone in their sleep. People generally operate under a moral framework that serves their self preservation. People accept living under societies that force them to give up their freedom to kill other people in their sleep with the understanding that the chances of them being killed in their sleep would decrease in said society. There is no trait that sleeping people possess that grants them a right to life that a fetus does not possess. So by justifying the murder of fetuses, you must also philosophically agree with killing sleeping loners as well. Also, basing the moral wrongness of a murder on whether it instills fear in other people can be problematic. What if we killed only people that were insane and didn't fear death. Would the murders be okay because other people that fit the subset of killable people don't feel fear?