Bugs are small. Bugs can be annoying. And bugs are easily squishable, given the right tools. So what better way is there to insult an opponent you don't want to be bothered with anymore? By comparing him/her/it to a bug (or worm, or maggot, or something else that crawls along the ground and can easily be stepped on), of course. Note that some Worms are less pitiful than others. See also the Drill Sergeant Nasty, who uses this as a "motivational" tool ("You call that an insult, maggot?!") Examples of Pitiful Worms include: