Maybe because it was an honest mistake? People make honest mistakes that end in awful ways (like this) it doesn't automatically make them a terrible person. I've seen bad calls being made- I'm not going to lie, I made a pretty bad call a few years ago (which I'm not going into) it was an honest mistake which had serious consequences- I know that I'm not a terrible person though. It's called human error, it happens.
If she was always beating the piss out of her kid or just abandoning them whenever she felt like going out to party or whatever, then yeah- that's a terrible mom, but she obviously had a reason to trust this guy to take care of her kid. Like I said, parents generally don't leave their kids with people they don't trust and expect to take good care of them.
Life isn't as black and white as some people like to make out.