When the red ant slips inside the turtle's shell, the turtle withdrew its head, and limbs into the shell, thus crushing the ant.
The turtle stopped its voyage to rest, when all of a sudden, an ant crawled inside its shell, which bothered the turtle. This is why it decided to kill the ant, and just keep on walking afterwards, as if nothing happened at all. The ant is not mentioned anymore in the story because it is irrelevant.
