Multiple reasons - but mostly because humans have a genetic ancestry of killing to survive - it's wired into out brains - but we have evolved socially to create culture and the rule of law, as a check on homicide. Anthropologists have recently discovered that humans have over large adrenal glands needed for survival and underdeveloped prefrontal brain lobes, which controls impulsive, often violent thoughts. Human males also have an abundance of the hormone, testosterone, which is also linked to aggressive behavior - often ending in murder and war - we needed this aggression when we were struggling to survive before culture, when we were still in the food chain. These three legacy organs and organelles work together to make humans the most dangerous creatures on the planet.
It could be self defense or someone got caught up in the wrong crowd like a gang and made the wrong decision or because of greed or beliefs or because of a mental issue or because they are just plain evil and do horrible things for no other reason other than they were born to be an evil person with the Devil inside them.