I've been discussing this at length on another forum. It's a regrettable situation, but the cops are in a tough spot here. They're given a warrant that basically says this guy is "likely" to be dangerous, so they go in wearing full gear, safeties off, to make the arrest. They had reason to suspect the guy is a drug dealer, but that'll have to play out in the courts. The dog was barking, and I don't know if the cops ever acknowledged that they knew it was in a cage when they shot it. I would guess they didn't know since the lights were off and they were using flashlights, but who knows.
A lot of the outrage over this is Monday Morning Quarterbacking the cops' actions, and that stinks. If you're that cop, you've got a split second to make that call, and dogs are dangerous in that situation because while the cops are distracted by a dog attacking one of their own, the bad could come around a corner and start shooting, then you've got a bunch of wounded cops. So you have to take care of that threat right away, and move on with your goal of finding the bad guy.
What I find far more sad is
the little girl who got killed in a similar raid in Detroit just this last weekend. Seven years old, the police break in to arrest a murder suspect, a woman in the house goes after the cop, a shot is fired, and she's killed. Horrible.