They weren't killing anybody though, they volunteered to be sacrificed to save their society as a whole. After the split, they do not see what they are doing now as actually killing anybody either. All life as it is now are as Emet put it, malformed creations. Everyone from the shards is just a piece of someone from the source's soul. Rather they are pieces of what was once a single soul of what ascian race was, as shown when Emet thought he saw the WoL as their 'original' self.
The WoL is hinted at once playing an important part in the past before the split, Emet recognized them. To clarify they do not see the people of today's world(s) as people. They want to sacrifice them to make them whole again, as they once were. They are thinking on a grander scale, we are talking souls here not the mortal bodies. Unfortunately, even if they did successfully unify all the worlds into the source, and bring back the old. The same thing would just happen again.