In a lot of films, whenever someone dies, it almost-always shows them lying with one leg bent at almost a 90-degree angle.
Why are the bodies depicted like that? Is this some old rule or an unsaid agreement between directors?
It's what audiences expect to see.
They expect the victim to collapse on the floor in a generally untidy state, and stay as they fell.
Lying down neatly with arms and legs straight looks unnatural to most audiences.
However, in real life, the nervous system and muscles do strange and unusual things to the body (depending on the mode of death), and this isn't the kind of realism that audiences typically know about or wish to see.