So this narrative that the kid was telling his mother he was going into the cage is just hearsay? If so, then it doesn't belong in rational discussion, and even if that were the case, 3 and 4 year old children say all manner of ridiculous things all the time. If you took every threat a small child made to heart, they'd be gagged and chain like hannibal lector in silence of the lambs 24/7.
Facts are, the animal was dragging this child around like he was a rag doll, the experts that have lived and worked with this animal (presumably for some 17 years of the animals life) made the call to terminate said animal. This couldn't have been an easy decision, but it was obviously the necessary one or else they wouldn't have made it, and NO, they didn't decide to shoot the gorilla for fear of a lawsuit ffs.
News reports claim the mother of the child was at the zoo with said child plus three others, those that can't appreciate how fast something like this can happen with a small child don't have children and don't appreciate how quickly a child can get away from you. This isn't her fault, the kids fault, the gorillas fault or the zoo, this was a freak accident.
And to those who value the animals life over the childs, go live in the jungle with some great apes (who coincidently have a reputation for killing the babies from rival tribes by ripping them apart and smashing them against rocks, look it up!) then because we don't want you in our species anymore.