Yes, I agree with you. The ex-partner was apparently abusive but you're right - she's just a particularly nasty human and we're all so far on the other side of the spectrum that we literally can't fathom it.
But I think this - if the ex-partner was abusive she may have channeled her aggression onto the child - she couldn't win a fight against him so she had a vindictive pleasure in abusing his child as a distorted coping mechanism.
Despite what others are saying I imagine in the early days she hit the baby when it cried, shoved her hand over it's mouth. When people do acts of unthinkable nastiness it starts with "dehumanizing" - hence her saying this child wasn't part of the family.
A reason for women to kill their babies is often as revenge against the man - I think this was a similar thought process.
Unfortunately the first 4 years of a humans life is extremely important, and this child has lost 2 years and 11 months of that process. Even if the mother gave a good explanation and I had some sympathy, what I think is this: abuse of this level is effectively a life sentence and she should receive a life sentence in return. Whatever she may have gone through, I'm sorry for it, but there is no excuse.