Correct me if I'm wrong, but I believe you need to have actus reus (guilty act) and mens rea (guilty mind) to prosecute. Therefore, in addition to intent, the prosecution would have to show that throwing bubble gum on the sidewalk was an act that could lead to murder / lead to accessing a computer system without authorization. In that case, all this talk about what is/isn't authorized access still complies with the basics of common law because it defines what is an actus reus.
Unless it's a strict liability crime. In the bubblegum example, I believe you could be convicted of something like manslaughter without mens rea. The CFAA is not a strict liability offense, however, as far as I'm aware.
You do have to have an act, to avoid punishing thought crimes, but intent is where the real action tends to be. Intent can make the same act (killing a person) anything from murder to no crime at all.
IANAL either, but I don't see how incompetently trying to kill someone using a method that couldn't actually kill the intended victim would get anyone off the hook.
Disclaimer: IANAL either.