So sorry it happened. It's hard when one of our pets kills another one because we do sort of feel emotionally that we can 'trust' our beloved pets not to do things that would make us unhappy and when it does, logic doesn't stop that hurt feeling. I mean, we know intellectually that the cats are hunters and would kill the snake because of instinct, but emotionally it's like a betrayal because you loved both pets.
Yes, you "should have known better" but then hindsight is always 20/20 too.
When you have a rack, it's fairly easy to drill a small hole in each shelf and thread a thin metal "pole" down through all the holes to keep any of the bins from sliding forward. I've seen some racks like this to keep small children from opening them and it's always struck me as a simple and effective "lock".