The way you stated your question, I'm not entirely clear what you are asking. You seem to be asking, "By what means/under what circumstances can someone simultaneously justify taking someone's life and upholding life value (right to life)?"
It is technically not a conflicting position to hold as long as you don't believe that this life value (or "right to life") is an intrinsic value or right. You just have to assert that it is contingent on each person's behavior. That is, one's right to life can be forfeited under certain circumstances (i.e. after a heinous crime is committed).
Under this view, people are born and by default they are given the right to life as long as they abide by society's prevailing moral standards.
- A fetus then—which has not made any real choices and is thus innocent—has a right to life.
- A criminal who is facing the death penalty has summarily forfeited his right to life.
It is only a conflict if you hold that life is intrinsically valuable. Then you would never be able to justify taking a life, except perhaps in self-defense.