If you saw someone murdering their baby after they were out of the womb, that wouldn't be okay. Why is it okay in the womb?

Easy, because inside the womb that baby is actively using the body of the mother, therefore she has sole consent and decision-making power over who can or cannot use her body. If she doesn't want this baby to use her body against her consent, then she has every right to stop it from doing so, by any means necessary. Outside the womb, it is not using anyone else's body, which is why it wouldn't be justified to kill it then. It's about consent and the use of other's bodies.