Or, we can simply say that if a gene makes a male mate with 2x as many females but also makes 20% of the females die with the child dying too, it still propagates.

Propagates for a very short while. If you initial population was stable (which means that each female had, on the average, two children which survive until they breed), introducing a mutation which kills off 20% of the females during birth is likely to lead to this population dying out pretty quickly. Yes, you'll have lots of males around, but they can't give birth.