In most cases of dog aggression towards humans I would certainly place blame at the door of the owner. Lack of proper socialising and training can have a detrimental effect on any animal, then coupled with a powerful breed such as a Rottie this can lead to disaster. Dogs are animals and as such have no concept of right or wrong, they have no conscience or morals, therefore in any incidence of dog attacks, the blame should generally be placed on the owner. Rotties, and Kerries for that matter, should be absolutely reliable with people, most notably children, they should NEVER display any type of aggression towards humans, both current Breed Standards call for this. Kerries however should exhibit a disciplined gameness (as required in the Breed Standard), with regards it's working role, but this must never be coupled with aggression. Historically Kerries were kept within the family home or farm, and as such unpredictable or aggressive dogs would be culled as they were not fit for purpose.
A well breed and well trained Rottie is a delight to own.
Dog on dog aggression should never be compaired with dog to human aggression. There is no research to support that a dog that is intolerant of other dogs, as is occasionally the case with both Rotties and Kerries, will ever display aggression towards humans.
PS. Hope your Kerry is doing well and giving you lots of joy!
Edited to add: neither of the Breed Standards, Rottie or Kerry, have been significantly changed for years.