> What does "attrition" mean? People killed in car crashes?

Yes

> How could that not be a net loss?

Is every human a net-positive to society? Obviously an objective measure is impossible to establish, but I doubt everyone is a net-gain.

How random are car-related deaths? Is this actually a problem?

In general I think society is based on the idea that everyone's life has inherent worth, and thus their rights should be protected. If we start designing systems based on the idea that some people's lives have negative worth and if they die it'll be beneficial society, that flies in the face of the idea of inherent worth. It seems to me that if society says certain people's lives have negative worth so their lives don't need to be protected because their death will benefit society, that could progress into society saying that it's ok to actively kill those people.

Yes, the US does today in fact actively kill some people with the death penalty. Though it does have a lot of checks in the process to avoid making the determination wrongly. It's a very expensive and thorough process. I'm against the death penalty because it goes against the idea of the inherent value of human life. If you live in a society where jails are very primitive and can't stop determined prisoners from escaping, then the death penalty could make sense to prevent prisoners escaping and then hurting others. We have jails that can hold people though.

By and large, people very much like to stay alive.

So, if you don't care much about Bob one way or another, but Bob and his family care about Bob, on net society cares about keeping Bob alive.