It's a combination of negligence by the zoo (having enclosures that are accessible by visitors) and yes, I'm sorry, but the parents have lapsed with supervision.
I don't believe they deserve to be flamed, but they have let their boy out of their sight long enough for him to climb out of his stroller and into the enclosure. There was also a quote in the article about the boy saying he wanted to get into the enclosure - if my son said that (which he probably would - he has ASD and very little sense of danger), I would be holding onto him within a safe distance to ensure he didn't follow through with his dangerous plan.
I feel super sad that the gorilla had to die, however I think the zoo had to make that call unfortunately. I'm glad they were able to rescue the little boy.