When Waaagh! Bork rampaged out of the Ghoul Stars and into the Eastern Fringe, the Wyrm-Killa tribe came with them. Festooned with alien trophies and covered in scars, this Snakebite tribe earned their name killing Tyranids.
Runtherds tell tales of how the tribe was hunting through the jungles of some remote world when they came across a great fleshy spore encased in glistening amber. Dragging it back to the Ork fleet, they cracked it open and hundreds of clawed, serpentine Tyranids spilled out. The Snakebites hunted the creatures, taking great pleasure in the fact that they seemed to grow rapidly, and no sooner had you bashed one than another one appeared from a vent or drain. Before long the Ork Kroozer was infested, and most of the other tribes on board were killed, only the Snakebites surviving in the weird Tyranid jungle their ship had become.
By the time the infested kroozer drifted into a system, the Snakebites had finally killed most of the Tyranids on board, having learnt the best ways to crack them open and smash them up. Covered in Tyranid trophies and using alien claws and talons as weapons, they joined up with other Orks, calling themselves the Wyrm-Killa tribe. Having had so much fun hunting the Tyranids in space, the tribe had kept some alive and released them on the first world they came to. While the rest of the Orks bashed the human inhabitants the Wyrm-Killas made sure the Tyranids had time to multiply, so they would make good hunting. By the time the Orks had torn apart the humans they found themselves overrun by Tyranids, and gleefully turned their attentions to this new foe. Since then the Wyrm-Killas have spread Tyranids to dozens of worlds so they can have a good fight wherever they go.