Cancerous tissues are cells that are so alive they multiply uncontrollably, and for the first half of a pregnancy, the child has no ability to represent being alive or even have consciousness. When a miscarriage happens, the child is already dead and abortion is saving the life of the parent. When rape happens, abortion saves the mother from major trauma. In certain cases of alcohol or drug usage, or just pregnancy protection failure/misuse, the child is a mistake and can cause severe damage to the life of the mother, such as falling below the poverty line and life being very hard to survive, if it even can be.
Some people lose their job, their house, and end up living in their car on the streets because they had to feed children they were unprepared to support. Is this better than killing a tiny ball of cells before it can be properly classified as alive?
Up until birth, a child's actions in the mother's body is nothing more than that of an organ, showing no real signs of life until exiting the womb.
If the child is not developed to the point where they can exit the womb and survive without being directly attached to the body of another, they are not, by the scientific definition of life, a living being, and it is therefore not murder.