No, you're not paying attention. There are 3 independent variables, negligence, intent and threat, and the various permutations of these are what define the classes of murder, criminal negligence, and so on.

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.