The reasonable belief is, as was clearly stated, decided by the shooter
What a "
reasonable belief" is or is not will be decided by the Jury, if the State decides to prosecute. I note that the first part of the statute simply defines when someone can lawfully use "physical force". The use of "deadly force" is stated in the negative (
a person may not use deadly force...unless...) in another part of the statute. That makes makes this a defense to murder. As a defense, the killer now has the burden of proof to show that he was justified in his belief and in the use of deadly force.
So using the scenario and Missouri law:
1. Is someone siting in a van who experienced someone throwing a plastic cone at the van the initial aggressor?
2. Did the guy who allegedly threw the cone withdraw from the encounter?
3. Was the deadly force used by the guy in the van reasonably believed to be necessary to protect himself against death, serious physical injury, or any forcible felony? Let's not forget that the dead guy was unarmed.
then, the legal system must prove "beyond reasonable doubt" that he is either lying about his actions based on his belief that he was in danger, or in some other way provide proof that the shooter violated one or the other laws of the state.
No. This is wrong. The State can and will prove beyond a reasonable doubt that the van passenger killed the passerby. He's admitted to this - the State's initial burden has been met. Now, the Defendant has the burden of proving his defense, i.e. that the killing was based on a reasonable belief that he was protecting himself from death or serious physical injury. This is HIS burden of proof. And, all the state has to do is sit back and try to poke holes in it.
I think a prosecutor can make a plausible case against the guy in the van, under Missouri law. Who knows who wins, but after spending $100,000+ in attorney's fees, the Defendant will have his answer.