I think it is a form of judicial corruption where the justice system does what is popular rather than what is right. Pretty much everyone drives and anyone can make a mistake while driving. The needs of the majority outweigh the needs of the few.

The Canadian Supreme Court actually stated this principle in a decision where they weighed the "social value" of driving against the responsibility of the driver. If we were to make drivers strictly responsible for their actions we would make the value of the automobile less.

Ref: R v Roy

Unfortunately, making the automobile worth more, makes pedestrians (ie everyone's) life worth less.

I'm betting that's because it's at the bottom of a list of deaths whose cause can be directly traced to an individual.

Surely the perfect murder would look like an act of complete provenance, with no apparent ties to anyone else or suspicions as to the cause.

Depends on what your goal is. Some people would gloat a lot, if everyone would know they did it, but nobody could prove anything in court.