Contrived strawman aside, this is exactly why there are different legal definitions for murder in the 1st, 2nd, and 3rd degrees, as well as manslaughter. And different punishments for each based.

In this example, it's probably open-and-shut 2nd degree but the prosecution now has to prove that you premeditated this if they want to go for 1st degree (IANAL). They don't get to just say "it was clearly premeditated" and then put the onus of proof on the defendant.