Alligators are present in almost every lagoon, pond, lake or body of water and I've even seen them in the ocean when I lived in Cape Canaveral. They will travel miles to find water and you will generally not know they are tbere.
I don't know how Disney would be at fault when gators are indigenous to the area and they do remove alligators as they're found. You can't expect Disney, or any other entity for that matter, to keep 24 hour quard on all areas where wildlife will exist even when human animal interaction can occur.
Yes it is tragic and I feel pain for the family, it's a tragic incident but unfortunately in today's "I'm entitled" society Disney will pay.