As any good law student can tell you, to be convicted of a crime, the State needs to prove beyond a reasonable doubt that the defendant committed an actus reus (guilty act) with the requisite mens rea (guilty mind). In other words, the State needs to shows that the Defendant had a particular state of mind in committing a crime.
Perhaps the best example of the importance of the mens rea requirement is the distinction between murder and manslaughter. In Missouri, murder requires the the Defendant to have acted knowingly after deliberation upon the matter. Pre-mediation is required. Murder carries with it the highest criminal sanction: a Class A Felony.
Manslaughter, however, is a lesser felony. While the actus reus is the same as murder, the mens rea is that the Defendant acts out of a sudden heat of passion or knowingly assists another in self-murder. Voluntary Manslaughter is a Class B Felony. Involuntary manslaughter generally arises out of a general reckless disregard for the life of another.
Therefore, it will be the job of the prosecuting attorney and defense attorney to show facts and evidence which tends to reflect what the Defendant was thinking before or during the commission of a crime.