Homicide is the unlawful act of taking someone’s life, otherwise known as murder. It is generally regarded as the most serious of all crimes, punishable by lengthy prison terms up-to-and-including life sentences and, in some states (including Missouri, but not Illinois), death.
To secure a homicide conviction, the prosecution must prove the elements of the crime beyond a reasonable doubt. That is a high bar. The prosecution cannot simply show that it is more likely than not that the accused murdered the victim. Instead, the prosecution must present evidence sufficient to put to rest any reasonable doubt in a juror’s mind about the accused’s guilt.
Degrees of homicide
State criminal statutes typically enumerate several distinct degrees of homicide, differentiated by the accused’s level of intent to kill the victim and/or the specific criminal acts that contribute to the victim’s death.
First degree murder
In most states, including Missouri and Illinois, homicide committed with premeditation and intent to kill constitutes first degree murder, which is the most severe form of the crime punishable by the highest penalties. In Illinois, it also constitutes first degree murder to cause another’s death while knowing one’s actions create a “strong possibility of death or great bodily harm,” or to cause a death while attempting or committing a forcible felony (other than second degree murder).
Second degree murder
Missouri, along with most other states, defines second degree murder as causing death while having the intent to kill or cause serious harm, or while committing a felony, but generally without premeditation.
Illinois, however, includes unpremeditated murder and felony-murder in its definition of first degree murder, and defines second-degree murder as committing first degree murder while acting “under a sudden and intense passion”, or while unreasonably believing the killing is justified.
Voluntary & involuntary manslaughter and other lesser homicides
All states also have criminal laws on their books defining forms of homicide in which the accused did not intend to kill, but nevertheless caused a death through reckless or negligent actions. These statutes usually refer to this form of unintentional killing as “manslaughter” or “negligent homicide”.
The prosecution (ideally) charges based on what it believes it can prove beyond a reasonable doubt
In the wake of an unlawful killing, prosecutors must decide which form(s) of murder with which to charge the alleged perpetrator. A fundamental consideration in that analysis is (or at least should be) whether they believe the evidence shows beyond a reasonable doubt that the accused intended to kill (justifying a first or second degree murder charge), or merely engaged in dangerous actions that resulted in death (justifying a manslaughter charge).
Proving intent, especially intent with premeditation, can be difficult. As a result, prosecutors often charge an alleged murderer with not just the greatest degree of homicide they believe they can prove beyond a reasonable doubt, but also several lesser degrees of homicide the evidence may also support. This is a standard practice, if used responsibly. Prosecutors have occasionally been criticized, however, for including difficult-to-prove charges with potentially lengthy sentences in a case simply as a means of pressuring a defendant to plead guilty.