I felt from the beginning that the girlfriend's account was questionable. She says that after the officer stopped them, he asked for license and registration, then shot him when he reached for his wallet. However, before that, she says Castile told the officer he had a gun on him. When a police officer is informed that a person they have stopped is in possession of a firearm, they do not then ask for license and registration. They immediately tell the person to freeze, then they disarm the person, then they may ask for license and registration. No officer on the planet would have asked for ID after learning of a gun without first disarming the man. Her story doesn't add up. What's more likely is that the officer asked for license and registration first, before being aware of the gun, then when he saw (or was told) about the gun, he immediately told him to freeze. Castile then reached either for his wallet or the gun, and the officer shot him.
In any event, because the video doesn't show the actual incident, it's a he said, she said, which means reasonable doubt.