If the person had known they would have taken a life, I'm not sure they would have stepped in, in the first place, knowing they will go to jail. Hence giving the original offender, and all others potentially, the balance of power.
Dangerous driving: person wearing dark clothing walking on the side of the road, driver is tired and only had 1 alcoholic drink 45 minutes ago but only 5 minutes from home. So tired that they need to take an extra second to check that the bush on the right is not a kangaroo about to jump out that they inadvertently drift off the road slightly and hit the pedestrian that was walking home from the pub on that unlit road.
My point is there are always going to be blurred lines and masses of grey areas that leave canyons of room with plenty of crevices for interpretation, and as with all new laws there will be a need to review and make amendments occasionally.