What made the otter killed the dog?

Otters are generally not aggressive towards dogs, but they may attack if they feel threatened or if their territory is encroached upon. In some cases, an otter might perceive a dog as a threat to its young or as competition for food. Additionally, if a dog is playing near water where otters are present, the otter may respond defensively. It's important to remember that such incidents are rare and often occur under specific circumstances.