Jesus told us exactly the condition of the DEVIL from the beginning. It was never a question of an Holy Angel going bad from making some bad decisions:
John 8:44
44 Ye are of your father the devil, and the lusts of your father ye will do. He was a murderer from the beginning, and abode not in the truth, because there is no truth in him. When he speaketh a lie, he speaketh of his own: for he is a liar, and the father of it.
Those are the PERFECT WAYS of the DEVIL. The term 'perfection' has little if anything to do with PERFECTION as in PERFECT other than PERFECTLY EVIL.
As it pertains to this topic the 'whole world' is presently immersed in the mystery of iniquity. It is in fact an unseen working, therefore termed a mystery.
All of us, adults, babies, everything in our present deteriorating world are in fact quite immersed into this working of EVIL.
1 John 5:19
And we know that we are of God, and the whole world lieth in wickedness.
Yet God has chosen HIS LIGHT to SHINE from the midst this present darkness.
Asking then if babies are 'sinners' is quite a simplistic notion. It is perhaps a little more complex than to merely see babies as that is not an 'encompassing' view of sin or evil.
When discussing matters of sin and evil, those present powers and 'entities' that extend upon the whole world must be firmly in view.
Do I see evil babies? Uh, no. God saves them all. They like us however all have the same issues when it comes to the present working of the mystery of iniquity which is not 'divided' out upon any individual in particular, but upon every individual when we consider the larger picture.
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Well, I see you refuse to see how the one guilty of bringing sin into the world was once highly favored of the Lord until iniquity was found in him and was cast out of the Throne Room of God and cast to the earth to destroy the nations.
I see no evil babies either as they are only subjected to sin when they enter this sinful world and eventually sin at some point in their life as we know that all have sinned. There is no mystery to iniquity, but an actuality as being that of Satan whom is the father of lies who spreads his deceit through out the nations causing man to sin, even if they are not aware of the source of sin.