Not trying to be difficult here or to cause trouble, but this is an example of how difficult it is to resolve certain kamma issues.
Suppose there is a train driver driving a train along a track. He comes to a fork in the tracks with the possibility of going down two different routes. In each of the two forks, unfortunately there is a known Arahant monk in the community. Should the train driver hit the Arahant on the left track, or the one on the right track? Or should he derail the train altogether, killing himself and hundreds of passengers on the train, causing much grief and lamentation to their surviving families?
It could be argued that the Arahants’ deaths would be a final release for them from suffering and thus would not matter much for them. But death would not be a final release for the hundreds of people on the train, and could cause much further suffering down the road. Furthermore, to the train driver, would hitting one Arahant in order to save another Arahant be considered an anantariya akusala kamma?
In the case of whether or not to put parents on life support, I view it as seizing the opportunity to save and preserve the lives of the parents. It should be a kusala kamma. Please correct me if you think I am wrong.