If you destroy someone else’s kidney’s, I think you have a moral/ethical obligation to them to try to restore them to good health.
If they lose only one kidney, then to me, that would mean you should be responsible for the medical bills needed to restore them to proper order–it is my understanding people can live long and healthy lives with just one kidney.
If you destroy two kidneys, to me that means you need to do everything in your power to restore at least one kidney to them. If that kidney cannot be found from a donor, then you should offer to give one of your own (if they are compatible.)
That is what I think your ethical obligations are, however, I believe that despite those being your ethical obligations, it would be unethical to force someone else to do give up a kidney.
I do of course, think if you stab someone and they lose both kidneys and die from lack of transplant, you should be prosecuted for murder with all the penalties that suggests–and I imagine a lot of people who are facing life in prison or a death sentence might be willing to give up a kidney to keep their victim alive since they would then be facing significant jail time but not life in prison or the death penalty.