The key word is separate. If someone kills someone through negligence or intention they're liable.

Now, if someone acts in a risky situation and kills someone rather than saving them, they can be OK. But in those situations, it has to be a sudden problem that comes up or the actor has to get "informed consent".

Someone who, unleashed a gas into the atmosphere that cured many people of disease but also killed a smaller number of people, would certainly be prosecuted (and, sure, there's a certain of HN poster who doesn't understand this).