Why and where do red blood cells die?
To take the easiest bit first: Red blood cells are destroyed because lacking a nucleus, they aren't quite as powerful as other cells when it comes to self-repair and longevity.

As for where; it really is all over the body. They are swallowed up (literally) by macrophages lining the blood vessels, which are particularly concentrated in the liver, spleen and bone marrow, but present everywhere.

As they age, they deform, and the macrophages are capable of recognising this deformation and reacting to it (by eating the red blood cells).

When the macrophages have eaten and destroyed the red blood cells, the nutrients they contain are returned to the blood plasma, and captured in the bone marrow for production of new blood cells.

(This pattern is common in a lot of systems, by the way: Even though the liver is the primary organ to break down toxins, for instance, most cells in the body has some limited capability for doing so; this is true of many functions.)



Ref:


http://bril2002.narod.ru/bllod56.html
https://en.wikipedia.org/wiki/Drug_metabolism#Sites