It's an accident because it's not on purpose. If you intentionally kill your ex with your car then people don't call it an accident.
Moreover, there are less than 5000 pedestrian fatalities per year in the US, and the reason there are that many is that US drivers drive three trillion miles a year. That's less than one pedestrian fatality for every 600 million miles. At 12,000 miles a year it would take more than 50,000 years before the average driver would kill one pedestrian. And that isn't doing any accounting for the cases where the pedestrian was at fault.
Most drivers never kill anyone. And that remains true even though almost all drivers are idiots at least some of the time. Because to kill someone you have to be an idiot and be in exactly the wrong place at exactly the wrong time. But you can't control random chance, which is why we have insurance.
"Dangerous" is one of those words that have been so abused it has lost all meaning. Infiltrating a violent criminal organization is dangerous. Explosive ordinance disposal is dangerous. Driving 11MPH over the speed limit is completely mundane.
It is obviously possible to drive very dangerously, but people also have a pretty good sense about these things and therefore don't. The number of people who are stupid enough to drive 120MPH in a residential neighborhood is small.
It is also true that e.g. driving faster can be, in a relativistic sense, "more dangerous" than driving slower. But that is true regardless of the point of reference. Driving 10MPH can be "more dangerous" than driving 5MPH. Driving 5MPH can be "more dangerous" than not driving at all. But that doesn't make driving 10MPH (or 40MPH) "dangerous" in an absolute sense. It just puts it in the same category of weighing costs against risks that government engineers do when deciding whether to build a sidewalk or catwalk or traffic light, or pedestrians do when deciding whether to cross at a blind intersection or walk to a different one. Each of which is a "direct cause" when a driver traveling at 40MPH hits a pedestrian crossing at a blind intersection with no traffic light.
Moreover, the word "accident" does not in any way imply that the result could not have been avoided if different decisions were made or more care were taken - that is almost universally the case for "accidental" things.
