I agree with the others, @Katt- it was not your fault and you should try not to blame yourself (I know it can be hard after a tragedy). It is very normal to see your pets and loved ones in visions and spellwork because they are a huge part of our lives! The situation with Taz happened so suddenly and there is still hope.
Sending so much love and light your way 
I have been reading blogs from a very qualified dog behaviorist (she is suppose to get in touch with me) and apparently what happened is very common and just survival instinct of a dog. A younger healthy dog will kill an older (and maybe ill) older dog to draw attention away. It could have been a smell, a limp, or something us humans cannot pick up on. I know I should not blame myself but it is so hard. Lots of “what if’s” that could have prevented it. But according to the expert nothing in the world could have prevented it. Just wish the neighbours had minded their own business and had not called the police.