It is sad, and this is a person who does not need to be in any type of law enforcement, and certainly should not have a carry permit. You have to wonder if he would treat people the same way in his job. Definitely needs his gun taken away.
However, it is legal in the State of Maryland to shoot and kill (without liability) any animal that is threatening your person, livestock or property. This is probably why no charges were filed initially.
Edit: I also don't think this fits the definition of cruelty under Maryland law. I will have to look that up.