Primarily people die because of sin, that came from Adam and eve disobedience in the garden of Eden, death was the consequences they have to pay.
God gave warning to Adam and eve that if they eat from the tree that it will bring death to them.
Because of their choice to disobey it separated humanity from God who is the source of life which brought physical and spiritual death into the world.
Romans 5:12 said sin entered into the world through one man, and death come through sin and from this way death come to everyone.
Death is the wages of sin.
Jesus that was sinless died and rose again conquering sim and death.
For the believers death is not the end but a transition to being together with the lord.
God promised us a future were death is swallowed up forever.