I don't know much about shih tzus, but, it sounds like the dog is suffering from separation anxiety. I don't think you have to worry about the dog attacking you if it suffers from separation anxiety. If they've told you the dog is agressive or a fear biter or wary of strangers or strange surroundings, that's different. But based on what you're saying, it sounds like separation anxiety. If you are going to be keeping the dog, are you going to be able to keep it in a crate when you can't be there with it? Dogs will be highly destructive when left alone if they suffer from anxiety. The owners should know this and should give you the information you need to know. I wouldn't leave the dog alone if I were you, and, if you have to, you may want to keep him in a small area.