I would probably try and find other victims of this cop and sue him individually. Let hundreds of thousands of dollars in costs come out of this department for this one idiot cop.
Somebody with computer skills should start up a website that keeps track of dogs killed by cops. It might cause some hesitation on their part if every department had to fend off a half-dozen lawsuits or more every time a dog was killed.
I think there is more than one out there already . Check out dogmurders.wordpres.com and there is a Facebook page, "Dogs that Cops Killed."