Former shelter worker here. It's quite possible the shelter didn't know the extent of the dog's aggressive tendencies. It's also possible they did. Dogs can behave very differently in different environments. There are no enforced standards for behavioral assessments in shelters. It really just depends on the shelter and how well staffed and how well funded it is. I'm sorry you had that experience. There are plenty of dogs in shelters that do great with other animals. My boy is an example.