Thing is - everyone in this thread is going to agree this is a fucking stupid and dangerous thing to do. 99.9% of people you could ask on the street would agree it was a stupid and dangerous thing to do.
The 0.01% of people who do things like this do them because they don't have good judgement making skills, to a literally fatal extent in this case. That's not normal stupidity, or depravity, or whatever. That's someone who has had a horrendous upbringing of their own with high number of ACEs, or is severely mentally unwell, or has an addiction issue, or are extremely poorly educated with neglect levels of experience in real life, or have a learning disability that precludes them from a typical level of judgement, or a host of other things. Or, indeed, a combination of these factors.
People don't make choices like this in a vacuum - we can't go, "Well they're a fucking terrible person" and call it a day. People who make such staggeringly poor life choices are doing so because of one or multiple contributing issues that make them unable to make good, rational choices.
I'm not for a single second exonerating the mother of this poor, poor kid. But it's just not as simple as 'she's a bad person'. 'Normal' people don't make decisions like this.