Actually, that's precisely how the law works: an act which satisfies the requirements of a crime is illegal. An act which doesn't satisfy the requirements of a crime, no matter by how little it doesn't satisfy those requirements, is not a crime. Laws can only work by drawing lines. You have to do that.
Good gosh, your understanding of the law is appalling.
If the world were really so simple, why do you think there are lawyers? If all one needs to do is apply some bright line test to the facts, what exactly do you think lawyers do? In reality, laws are general written rules that promote justice.
Now, each of those words carries a great deal of meaning, but I'd like to focus on just one: words. What is a word? It is a rough and ready encapsulation of concept. The concept itself has no name, no description. We do our best to use whatever words we have to explain what we're thinking. But we often fail to agree. And if you have trouble believing this, try typing in fifty words or less how you feel about your mother.
You could write fifty or maybe even fifty thousand words about your mother and I doubt they would fully explain just exactly your emotions. I certainly couldn't do it.
That's all laws are, though. They're just a collection of words that sort of mean similar things to some people. And that's why lawyers exist. We try to persuade others that our interpretations should be the basis for decision-making. And usually there's a lawyer on the other side trying to argue that the words mean something very different.
You cannot draw a line. You can say that certain things are definitely on one side and others are definitely on the other. Aborting a 5 week-old fetus using medication is definitely allowable. Shooting a 17 year-old high school student is definitely not. But the closer you get to that line, the harder that call is to make. Shooting a 5 week pregnant woman in order to kill her fetus is definitely not allowed. Withdrawing life-support for a brain-dead 17 year old boy is.
Time, methodology, intent - all of these things help put the determination on one side or another. But if you think that a bunch of words can ever fully and finally draw a line between right and wrong, you are deluding yourself.