Short answers, pulled from the story.
Homicide is an act in which a person causes the death of another person. It requires only a volitional act, or an omission, that causes the death, so a homicide can be accidental, reckless, or negligent even without any intent to cause harm.
Criminal homicide is divided into murder and manslaughter based on the state of mind and intent of the person who caused the death. First-degree murder is premeditated, intentional, and unlawful, while manslaughter involves either no intent to kill or a killing caused by a disturbed state of mind, and it normally carries a lesser penalty.
A United Nations Office on Drugs and Crime report from July 2019 documented that nearly 464,000 people worldwide were killed in homicides in 2017. Earlier UNODC estimates put the total at 468,000 in 2010 and 437,000 in 2012.
Not all homicides are crimes, because some are legally privileged and others give a full or partial defense to prosecution. Common examples include self-defense, justifiable homicide such as a lawful battlefield killing or a police officer protecting lives, and defenses based on mental incapacity or the defense of infancy.
From about 1200 AD through 1800 AD, European homicide rates from local violence fell by a factor of ten, from roughly 32 to 3.2 deaths per 100,000 people, and reached 1.4 per 100,000 in the 20th century. Most historians attribute the decline to a steady increase in self-control promoted by Protestantism and necessitated by schools and factories, not to economics or state control.
In 2020, firearms were used in 76.7 percent of United States homicides, followed by sharp instruments at 9 percent, blunt instruments at 3 percent, personal weapons such as hands or feet at 2.5 percent, and strangulation or suffocation at 1.5 percent. The overall firearm homicide rate that year was higher than in the previous 20 years.