Sophie, you are a very logical thinker, and good for you!
The fact that all babies go to heaven is something I will deal with in a moment, but first let me offer the idea that those babies in Bethlehem and its surrounds, and babies who are slaughtered all over the world are allowed to have this happen by God for the precise reason of horrifying us and showing us the depths and degradation of sin. Many can rationalize to themselves (which always shocks me...) that an older person being killed is not the same....quite....as a little one being killed. After all, the older person probably 'deserved' it one way or another. But when a baby is killed by some murderer, we are shocked and horrified -- and rightly so.
But unless we saw the horrors sin and rebellion against God led to, would we really have any concept of just how damnable sin is? I don't really think so. I think perhaps if we were to know the truth of the matter, it is far more a miracle more are not slaughtered. As I look in the Bible and at history, it is not the disasters and wars which astound me -- for we are like that, aren't we? -- but, rather, the escapes and salvations (in a worldly sense) which point out to me that God has not lost control!
We humans are, truly, the horrors of creation. HOW God could love us is beyond me. But He did and does, and we are each given the chance to respond to that love.
Now, why is it hard for Calvinists and others to believe that all babies go to heaven? There is most certainly Scriptural support for that. Paul presents it clearly in Romans 7. Here, as presented in the NIV (the family is still asleep and I don't want to make noise getting out the other Bibles, but do encourage anyone who uses any other translation to look it up in yours) is the passage I refer to:
What shall we say, then? Is the law sin? Certainly not! Indeed I would not have known what sin was except through the law. For I would not have known what coveting really was if the law had not said, "Do not covet." But sin, seizing the opportunity afforded by the commandment, produced in me every kind of covetous desire. For apart from the law, sin is dead. Once I was alive apart from the law; but when the commandment came, sin sprang to life and I died. I found that the very commandment that was intended to bring life actually brought death. For sin, seizing the opportunity afforded by the commandment, deceived me, and through the commandment put me to death.
Romans 7:7-11
What does Paul mean when he says that "once I was alive apart from the law" unless he means spiritually? -- for we know that reincarnation is not what he is referring to! And if alive spiritually, then not condemned to hell. "Alive spiritually" is defined by Jesus in John 17:3 as knowing the Father and the Son. Therefore spiritual death is NOT 'unconsciousness' in a spiritual sense, but, rather, separation.
And when does that separation happen to a person? Not at or before birth, or Paul would be talking nonsense above. It happens when one responds to the law with disobedience. So there are two points necessary for one to become dead spiritually: to know the law, and to respond disobediently.
When does the law become known? Not mommy's and daddy's laws of the house, but THE Law -- the law of God...
Not by an infant! And although we often teach, or should teach, our youngsters the Ten Commandments, please remember that there is a difference between reciting somethng and knowing it, and its meaning.
When does that knowledge actually happen? It probably varies with different people and I don't want to argue the point. But this 'knowing' is not simply a memorizing, but requires some kind of understanding. I can 'know' a song in French by repeating the syllables. But the meaning can escape me! Children memorize all kinds of things they don't understand!
So one must respond to a law one understands with disobedience. The point about sin nature is that there is a guarantee that we will respond with disobedience! As God told Noah in Genesis 8:21, all men TEND toward evil in their hearts from their youth. It does NOT say, in the Hebrew or LXX Greek, from their 'childhood', as many modern translations say, but from their 'youth.' Youth is considered, worldwide, to be after childhood and before adulthood, or roughly correlateds to the late teens and twenties.
So yes, there is a variation in the way we understand any age of accountability, but God puts it at about 20. This is probably a safety factor, for it seems evident to us that there are those under 20 who must be held accountable and those older than 20 who seem so immature we would question it. So let's leave it with God there, knowing that that age does have significance. It also has significance biologically, for it is not until the first two decades of life have passed that all the neurological and glandular parts have matured in a human being! It takes a full twenty years for our bodies to settle into maturity. That's simply a biological fact.
Paul says above that he does not know what sin is except through the law, and that when he knew the law, sin sprang to life and he died.
That knowledge was the key thing. Babies do not have that knowledge.
They are born sick with sin, but not responsible for being sick. There is a giant difference! They may be 'deformed' by sin, but they did not do that to themselves and are, in fact, at that point innocent victims of sin. They have done nothing yet, good or bad, to deserve either praise or punishment!
Now, there is something else which is a guarantee that babies and the little ones go to heaven. Look at that old favorite verse, John 3:16, and then keep reading:
For God so loved the world that he gave his one and only Son, tht whoever believes in him shall not peerish but have eternal life. For God did not send his Soninto the world to condemn the world, but to save the world through him. Whoever believes in him is not condemned, but whoever does not believe stands condemned already because he has not believed in the name of God's one and only Son.
Jesus died for all sin. Not just some. His work was complete. Men are NOT condemned because of sin. That was taken care of by our Savior. We are condemned for not believing!
What baby can refuse to believe? What little one can? They don't know enough -- they have not yet rebelled against God's Law. Despite natural tendencies towards selfishness or lying or whatever, they have made no conscious decision to reject Jesus or disobey God's law. So whether we look at it from a belief or a law standpoint, the children are safe in God's arms.
Getting back to abortion, that in no way excuses it in the slightest. Murder is still murder. And it is not the victim who is in danger of hell due to the murder, but the murderers themselves. Again, though, not because of the murder as a sin, but because they have demonstrated complete rejection of Christ in not just murdering, but in defending that murder as somehow 'right'.
Does God WANT all those little ones killed? No. Never. Absolutely not. He created them to live. And if we kick them out of life here on earth, they will still live -- with God in heaven.
But does God ALLOW those slaughters? Yes, obviously. And in part, at least, to show us the horror of sin.