A female pregnant through rape never took any consensual action that would result in pregnancy. The embryo is there against her will. Since she never agreed to carry the embryo, she is under no moral requirement to carry it to term. When a female is pregnant, her body is essential providing life support to the embryo, which could not survive without her body's resources. If the female consented to become pregnant, then she agreed to give those resources to the embryo and is required to do so. However, in cases of rape, she never consented to provide her body for the use of the embryo. Therefore, while it may be commendable for her to do so, she is under no requirement to let the embryo use her body. For instance, we might consider it good to donate an organ, but failure to do so is generally not considered wrong because people have a right to decide how their body is used. The same might be said in the case of pregnancies through rape. 
Rape is a crime committed by a person who should be punished for it. However, killing the child, who is innocent does not solve the problem, and instead punishes the wrong person.