LES. I am so sorry about Loki. I understand how you feel towards Duke at this time but that will pass. When I was a teenager I had let our dog, Buck, out and he killed our little kitten, MC, the same way. It is not something I will ever forget witnessing and the feeling of there being nothing I could do at the time. My heart goes out to you and your family. Duke could not help it, just as Buck couldn't. It is instinct to chase what moves/runs. We never had a problem with Buck after that, even though we had other cats, inside and outside(we lived in the country) so don't think that he will always be like this because that is not necessarily true, only you know how to read your dog to see if maybe something in him has changed towards cats, maybe he will only be okay around them inside as stated above(?). Again, I am so, so very sorry for your family having to experience this...prayers for your little boy to find comfort in memories of his late best friend, prayers for you to recover from what you saw and prayers for Loki to run freely chasing grasshoppers at the Rainbow Bridge.