How come someone can have an abortion but once the baby is born it's considered murder to kill it?:
Most countries where abortion is legal, as far as I know, accept it only during the first three months of pregnancy, where the foetus isn't even properly termed a 'foetus' but rather an 'embryo' (I think). If you've ever seen pictures, it doesn't look anything like a human yet and it has no brain function and thus can feel no pain and has no awareness of its existence (for anybody wanting to ask 'how can you be sure?', all I can say is that I'm not - this is only what I've read). Well before birth, a foetus has become very much a human being (though not one that can survive independently) and then it is a crime to remove it.

Please remember that very few people who have had abortions see the question as black-and-white as many anti-choice people do. Every one of them wrestles with their own morals and conscience in their own way.