After the Fantastic Four's first encounter with and defeat of the Skrulls, Reed Richards hypnotized the four invaders to turn themselves into cows, and to believe that they really were cows. He left them in a field in King's Crossing, Vermont. During the Kree/Skrull War, the Skrulls regained their memories and impersonated some of original Avengers, in an effort to force them to disband. Another of the Skrulls took the form of H. Warren Craddock, who was running for office. The Craddock-skrull was slain after being exposed.
The remaining three former cow-Skrulls were handed over to the Alien Activities Commission, who forced them to become cows again. This time, however, they sent the Skrulls to the slaughterhouse, along with the rest of the beef. The Skrulls were slaughtered and mixed with other, real beef, meat and distributed across the country. In the form of hamburgers, the Skrull meat was consumed by an unknown number of Americans. Some of the people who ate the beef proved immune to the Skrull DNA. Some died horrible deaths. A small number, were "infected" and developed metamorphic powers, along with progressive and terminal neurologic disease.
Ryder began recruiting others with the same affliction, to join together and wipe the Skrull menace from the Earth, so that they could never again turn into cows and let others slaughter and eat them again. Thus the "Skrull Kill Krew" was born.
During the Secret Invasion saga, Ryder and Riot are all that is left. The others have perished from the disease. They stumble upon the new 3-D Man and they gain a new member. Riot eventually dies at the end of Secret Invasion. That leaves Ryder and the new 3-D Man to continue the hunt and kill of all Skrulls here on earth.