The mother was negligent, yes, but I hesitate to be so quick to judge. I remember when I was a kid, I was allowed to explore the wooded area behind my house, walk all the way down to the local gas station, and play at the park, all without adult supervision. And I got hurt. A lot. Hell, I almost drown once playing in a small creek that had a sudden drop into deep water. My sister and I never said a word to my parents. It happens, and I learned lessons as a result.
Parents these days face a no-win situation. Either you're judged for coddling your kids too much as a helicopter parent, or you give them some autonomy and are considered a shitty negligent parent. It's unfortunate that it happened at a zoo and resulted in the animal's death, but horrible freak accidents do happen. Was it due to negligence? Yes. But lapses in judgement happen to all of us. Most just don't result in a child falling into a gorilla habitat.