That is not normal dog behavior. Normally dogs, especially ones that are members of the same social group, don't just attack one another, even if there is some sort of perceived weakness.
I don't know if you saw this happen, or just the aftermath, but you should know that dogs don't normally do that. From reading the question, I'm inclined to wonder if you saw the actual attack, simply because this sounds more like a wild animal attack than something most dogs would do MSM Can Improve Your Dog’s Quality Of Life - Canine Finds . There could be something wrong with them, either physically (rabies?), or in the way they were raised and socialized (fighting dogs?).
Dogfighting is unfortunately still a sport in some cultures, and the training that fighting dogs are put through is extremely cruel and rigorous and lengthy, because it goes so profoundly against a dog's nature to attack one another. They are also disinclined to attack humans.
There are exceptions, of course, many larger breeds can become violent to protect a human, canine, or feline member of their family. And with unsocialized, undisciplined tiny dogs, all bets are off. I've seen some well-socialized, well-behaved small dogs, but most I've seen have been aggressive. Nurture really does make a difference when it comes to proper dog behavior.
If I saw my dead dog hung up on a barbed wire fence, I wouldn't immediately think that my other dogs killed it.
If I had two dogs who attacked a member of their family group, I'd call the vet. If I, as a passerby, suspected that it was because they were trained to be fighting dogs, I'd call the police.