Death in the world of Avatar is an unfortunate, but still possible, happenstance. People die of injury, sickness, old age, and other more random instances all the time. Normal humans are just that; humans that can be injured and age the same as everyone else. Even the powerful benders are not immune to this; the greatest benders of all time have succumbed to age and injury; battles and forces of nature. While benders show some enhanced resistances to elements; they are not immortal and immune to injury. A fairly basic, but still extremely effective guidelines to justifying a combat kill is as follows: if it is lethal in real life, it is lethal in Avatar.
Now, there are some exceptions to this rule of thumb. Each is fairly obvious, and each comes with it's own balance; as they are the inherent balances of the 4 bending natures.
Firebenders are highly resistant to fire; to the point that they can only be burned by fire of equal or higher rank. However, they are extremely weak to high rank water and significant cold.
Waterbenders are impossible to drown, and all but immune to cold; however they lack the physical strength to resist earth and lightning causes double the damage.
Earthbenders are physically powerful and highly resistant to damage; however they are slow and if swept off their feet they are rendered nearly helpless.
Airbenders are extremely fast, but supremely fragile compared to other benders. The take extra damage from any kind of physical attack that hits them.