Why is a gunshot wound infection so deadly?

NotSoMuch: The severity of a gsw is related to the structures injured in the course of the bullet; there is nothing intrinsically dangerous about the bullet, which is actually sterile (in contrast to tv shows, we do not remove bullets unless they are in harms way). A wound infection is no more dangerous from a gsw than any other wound except for any foreign material present (clothing, etc).

Deep and tunneling: Due to narrowness and depth, gs wounds are harder to keep clean without surgery. Without proper cleansing and drainage of deep wounds, antibiotics alone often do not manage infections well.

Any wound: Infection can be deadly. The simplest of wounds can become infected causing death. It isn't the cause per se it is what is or isn't done afterwards that matters.