Why do people kill other people?

The reason why people kill other people is due to negative emotion. Love, peace, happiness, compassion does not lead to murder. Hate, fear, bitterness, anger leads to murder.

The reason why people feel hate, fear, bitterness, and anger towards people is because of the way they think. The absence of truth in the subconscious mind leads to murder.

What initially caused the absence of truth in the mind is experience which for many is too painful to mention.

People that have grown up in an environment void of love, peace, happiness, compassion are more likely to feel hate, fear, bitterness and anger. Exactly how this expresses is different depending on the individuals belief system

A society that punishes murder with the death penality will (as a generalization) have less offences than a society that rewards murder. (For instance, religious extremists, political extremists, etc will commit more murders than their fellow citizens)