It's measured by its conventional result. Ergo, if it kills you there is no need to call it any shade of grey--it's destructive, bad, and resulted in your death.
Karma is a constant force. Although it is not responsible for everything, it does relate to everything in our lives. This fluid concept, coupled with the notion of "grey-shades," presents a good understanding of karma.
All composite phenomena are impermanent. I think that conceiving death as destructive and bad can create a karmic impression that isn't necessarily true.
Best wishes to the cat, how is he coping with only 3 feet?
According to the Buddha, destructive karma is a cause of death, among other destructive fruits. Holding an extreme view of destructive karma, like all extreme view, does lead to impressions that aren't true. But karma does commonly cause death and it is wise to consider this. Many Buddhists say that they recognize karma and yet they seem to believe that nothing is actually caused by karma. Both the view that karma does not bring any affects, and that karma causes everything that happens to us, are extreme and therefore lead to incorrect impressions.
The kitten passed away yesterday. And I can confidently say that karma played a role in its very short and miserable life. I was chanting for him and my impression was that he wasn't able to avoid another animal birth. However as we are offering his body to others soon I'm hoping that yet more prayer will be able to help him in some way.
I heard someone once say that karmic effects direct life, their side-effects influence it, and prayer is mostly responsible for the rest. Prayer here meant more than just spoken prayer, but the force of prayer in general.