Categorical imperative
Thirsty Man believes that the categorical imperative is the Golden Rule.  To be an ethical person, Thirsty Man believes he must at all times treat others the way he would want them to treat him.   From the categorical imperative of the Golden Rule, Thirsty Man has adopted a maxim of "I will give anything I can to anyone I meet, if that person needs what I have much more than I need it."  Thirsty Man prepares to decide if he will drink the water that he wants to drink, or if he will give it to Dying Man.   Thirsty Man tests both choices by comparing them to his maxim.   He sees that it is imperative  that he give the water to Dying Man.  Thirsty Man gives the water to Dying Man.  Dying Man drinks nearly the entire bottle, but then he chokes on the last sip.  There is nothing Thirsty Man can do to stop the choking, and Dying Man dies.  There are many philosophies of ethics, and many philosophers who have very different opinions.   Some philosophers might say that it would have been ethical if Thirsty Man had kept the bottle for himself to drink.   It was his bottle to begin with and he could do whatever he wanted with it.  Other philosophers might say that Thirsty Man was ethically wrong to give the bottle to Dying Man because the water ended up choking the Dying Man to death.