It is not normal for a wolf/dog to attack a younger member of the order. The younger members are welcomed.
The baby was not part of the 'order,' the baby was an interloper.
The rottie had just "lost" its owners and was probably disoriented and upset. Then there is a new and unknown baby, getting all of the attention from the "new owner." This makes the rottie feel threatened, in that it will no longer be the top dog under the owners in the pack, but will lose that status and affection to the interloping baby. In the rotties mind (I'm guessing here) the baby threatened its status, and had to go.
IF the alpha female takes the young pup away from its biological parent and adopts it for its own, then the other members of the pack will protect it. If not, then the pup and young dog will be a rival and subject to fights for supremacy.
If the grandparents (the alpha male and female) had been holding the baby, there probably wouldn't have been a problem.
My wolf hybrid always tried to kill my other bitches puppies. In the wild in a pack only alpha's are allowed to breed.