Yes, but if you mentally deranged child ( so severely mentally ill that they were completely unaware of what they were doing) were trying to stab you to death, surely your Church would allow you to use lethal force to defend yourself?
Yes, and arguably removing the fallopian tube constitutes just that sort of ‘lethal force’.
What you’re not taking into account is that even when using lethal force, the method chosen is of moral significance.
There are all sorts of weapons, for example, that are banned for use by armies under international law. Certain types of chemical weapons being one. Of course it may help a war effort greatly if I can gas all the enemy troops to death, but such a way of killing them is held to be inappropriate for all sorts of reasons, so chemical weapons are banned.
Some types of death are appropriate, as I was saying earlier, for rats and snails, but not for human beings made in the image and likeness of God, however deranged or however much a threat they pose.
Of course there is also the fact that the fetus is in fact doing nothing whatsoever to the mother, essentially. It is merely existing, and it is this mere existence that constitutes the threat. Makes it very different to a case where one is directly attacked, say, by someone wielding a knife or gun.
Let’s tease this out a bit. Say I have a two-year-old child, and that there is definitely not enough food to feed the both of us, due to severe famine or whatnot. And that if I die of starvation, no-one else will be able or willing to look after the child and it will die too. In such a case, exactly as in the case of a fetus, the child is no direct threat, but it is true to say that its existence is incompatible with mine.
By your logic I would be justified in poisoning the child to kill it, so that I might live. Seriously, that is where your logic leads, because the only moral difference is the age of the child.