Why does the Bt toxin not kill the bacterium that produces it but kills the insect that ingests it?

Explanation

Bt toxin is produced by a soil bacterium called Bacillus thuringiensis. This toxin does not kill the bacteria because when it is present in the bacteria, it is an inactive and crystalline protoxin form. It becomes active and toxic when it is consumed by insects such as lepidopterand (amryworm), coleopterans (beetles) and dipterans (files/mosquitoes).
The activated toxin (delta endotoxins) binds to the epithelial cells in the midgut of an insect eventually killing the insect.