The simple answer is: a Wormkiller shot is a poorly hit tee shot. 

If you need a more in depth explanation, read on. 

To understand exactly what the Wormkiller is, first we'll go over the other versions of the dreaded tee shot.  Let's start with the 'Good' tee shot since most of you reading this probably haven't seen one.
   
A 'Good' tee shot is where the ball is hammered off the tee.  It soars through the air and lands over three hundred yards away.  It can be under three hundred yards but that's actually called an 'Okay' tee shot.  Notice that there are no worms killed when using the 'Good' or 'Okay' tee shots. 

A 'Wormburner' is a tee shot in which the ball is hit hard, but doesn't soar.  Instead it zips along just a few feet off the ground.  It looks bad but still travels a good distance.  Usually no more than two hundred yards.

A 'Wormkiller' is a tee shot in which the ball is hit hard but doesn't even leave the ground.  Instead it rolls along the grass.  The ball still travels an alright distance but not really.  You just think it's an alright distance because you suck.

The 'Wormkiller Shot' is very similar to the 'Wormburner Shot' - in that they both result in the death of worms, however, we believe the Wormkiller is a much more humane way of putting a worm out of it's misery, and we're pretty sure all worms are living in constant misery, so we're really doing them all a favor.

Don't believe me? 

Worms live underground.  How fun could that be?  They don't have eyes, which might be good because then they don't know they live underground.  Birds eat them if they poke their little heads out for a breath of fresh air.  Come to think of it, they don't even have heads.  They have two asses. 

Death by golf ball is an honorable way to go for the 'Two Assed' worm.