The next line of defense is commonly that the driver "has suffered enough," sometimes simply by witnessing a horrific death that (by implication) they were powerless to stop, or even by being forced to consider the possibility that their behavior or choices may have been connected to this death.
In circumstances where there is really no denying that the driver acted recklessly or irresponsibly, the claim is that this knowledge serves as a lesson. The driver now knows the consequences of reckless behavior or irresponsible choices, and will never do it again.
Many drivers flee the scene of the crash, because they believe they will be held responsible for it and do not want to face the consequences. (Some do it for a chance to sober up.) Reckless killer drivers who do this have compounded their crime of reckless driving with another, leaving the scene.
Unfortunately, some people are so upset by this effort to evade responsibility that if the driver is caught they treat that as the major crime. Conversely, they may be so relieved that a killer driver turns themself in, or chooses not to leave the scene, that they praise that and forget any reckless behavior.
This is all bullshit. We have laws specifying the penalties for killing someone with a car. They could use a little more tightening up in the evidence department, but they're pretty clear on the consequences.
To the best of my knowledge we do not say that reckless knife-wavers have "suffered enough" or that negligent builders have "learned their lesson." We do not praise incompetent gun handlers for not running away. We should not treat drivers any differently.
Actually, we also treat some incompetent gun handlers differently - we sometimes say they've "suffered enough" if they negligently kill their own children. In a post about this craziness, some guy named Greg Laden made an astute point:
Suggesting that the decision to hold someone responsible for an irresponsible act that has damaged another should be based on how the perpetrator of that act feels post hoc, extended more generally, means that the standard for punishment under the law is inverse to the severity of the crime. It is suggesting that the severity of the possible punishment be inverse to the seriousness of the crime because how bad one feels is proportionate to the severity of the crime.
Knowing you killed someone is not punishment. Realizing that they might still be alive if you had made different choices is not punishment. Staying at the scene is not accepting responsibility. Turning yourself in is not penance. As long as we continue to treat them as such, people will continue to die.