I cannot say whether the shooting was justified or not, knowing America, it is possible that the cop overreacted.
You cannot say? It's possible he overreacted? If the dog was aggressive, shooting it would be justified, but I'm going to go out on a limb and say that the example here would have mentioned it if it were. So it's probably a safe to assume that no, the dog wasn't aggressive or a threat. In which case shooting it, right behind the shoulder blades no less, with a pellet gun was not justified. A pellet gun is also generally not what you use to shoot a large, aggressive dog, as it's unlikely to stop it being a threat to you.
There's now law in Missouri, nor an ordinance in St. Charles, that says a dog must be restrained in its owners yard. It cannot, however, be "at large" off the owner's property. To kill or injure a dog, there needs to be a "reasonable apprehension of imminent harmful contact." As I said, the cop would, and should, have mentioned it, were that the case.
This is what I said earlier in this thread
Sometimes squaring up to someone rather than cowering/apologising is precisely what de-escalates a situation.
The deputy only "squared up" when the guy was trying to enter onto what I assume is the rear deck of the property. Maybe you neglected to hear that the guy threatened to shoot the Deputy around the 0:38-0:40 Sec mark in the clip.
When leaving the property he shouted "You fcuked with the wrong marine"
I cannot say whether the shooting was justified or not, knowing America, it is possible that the cop overreacted. But to pretend as if the owner was not negligent in firstly securing his dog or that he did not enter onto the guy's property in a disrespectful and aggressive manner is ludicrous.
Let me draw a scenario for you and I want you to tell me what is your likely response.
An angry guy enters onto your property with his camera pointed at your face and screaming obscenities. He threatens to shoot you and actually makes the first few tentative steps to enter onto your back porch. Your wife is behind you and your kids are inside. What steps are you going to take to get him off your property?
You shoot someone's dog, you better be prepared to have an angry person show up at your doorstep demanding to know why you did so, doubly so if your reason for shooting it was that it was allegedly in your yard, and not, you know, that it was actively trying to harm you, your family or your family's pet(s).
In any case, I'd let the guy vent, and I certainly wouldn't be antagonizing him further (what you gonna do? i'm proud (to have shot your dog), why wasn't your dog chained up,
buddy?). I've illegally shot and killed his dog for non-aggressively being unleashed in the vicinity of my yard, so I'm not in the right here anyway, and him coming to confront me was expected. I also wouldn't have expected his dog to have died from being shot with a pellet gun, so upon finding out I would humble myself appropriately. If his body language suggested he might resort to violence, I would try to maintain a safe distance between us, to give me time to react. The guy with the camera doesn't actually seem like he's looking for a physical confrontation, though, so it's better to just let him rant. I'd probably block him from coming onto my back porch, but otherwise I'd have done just about none of what the cop did in that situation.
Also, note that the cop claims he shot the dog for being in his yard. Owner says the dog was shot in his own yard, three houses away from the cop.