After pulling the plug on his dad it doesn't seem so bad. Keep in mind this is the same McCoy who had no problem shouting out his futuristic medical knowledge in a 20th century hospital, likely violating all sorts of Federation temporal laws. Even the woman with a new kidney might have her cure reverse engineered, causing all sorts of havoc to the timeline. Countries fighting over this would be enough to spark a Eugenics wars or World War 3. Oh wait... :O (And we thought McCoy saving Edith Keeler was bad).