What if I deliberately poison someone in an attempt to murder them, but botch it so that instead of quickly killing them it just damages their kidneys to point that they will die without a transplant, and I'm the only compatible potential donor?

Wasn't my wrecking their kidneys a breach of their bodily autonomy? Why shouldn't I be required to give up some of my own bodily autonomy to reverse much of the damage I did to their bodily autonomy?