So in the space of a minimal online discussion you've gone from 2 classes of murder to now 5 classes (3 degrees of murder + negligence + under threat).
Now continue this exercise for the next 250 years and you will eventually have a codification of the classes of murder which roughly mirrors our current laws. Which is to say, it will get progressively more complex.
3 independent variables is not a complex model. The complexity of the law that people bemoan are not about simple issues like murder, they are issues like zoning laws, tax law, and the numerous byzantine exceptions that are carved out to serve special interests, or laws that are archaic and have never been revisited or are only selectively applied.
So if I plan to kill Person A, but miss my shot and kill Person B whats the charge? What if I was under threat by Person A? Who should even be charged for Person B's death...me or Person A?
Moreover, you've neglected to mention that there are all sorts of "human killing another" which are not even considered a crime (abortion, do not resuscitate orders, etc).
>The complexity of the law that people bemoan are not about simple issues like murder, they are issues like zoning laws, tax law
Lets not get ahead of ourselves, we still have barely scratched the surface with the 'easy laws'