As someone else has pointed out, it was common for babysitters in the 1980s and earlier to be very young themselves. I have family members who were paid to babysit unrelated preschool-aged children for many hours straight at age 12 or 13 during roughly the same time period of this murder, so it's very plausible that the sitter here wasn't much older. If it was an adult watching the kid, absolutely, that's a fucking criminal level of negligence. If it was a young teenager, though? I don't think they ought to be tarred for life on account of a stupid decision they made at an age where everyone makes stupid decisions. I'd consider the guilt of knowing what they could have possibly prevented to be more than enough punishment in that case. Unfortunately, the lack of info on the babysitter makes it impossible to say just how deserving she is of our anger, so I'm going to take the safe route and assume she isn't, barring evidence to the contrary.