I dont think it is accuarate that Christians believe they existed pre conception. God is all knowing and exists outside of time. Because I know I will make a pizza tomorrow doesn't mean it exists today.

Age of accountability is debated but it isn't clearly laid out biblically, most people just assume it is a thing because they believe it is God's nature to act this way.

Better to be killed than to reach the age of accountability and possibly go to hell? Sure, its a better scenario for the baby. Christian doctrine is that it isn't about us but glorifying God, which all people do one way or another.