Dragon Initiation Test
Killer, along with his Master Thuggory and all the other dragons and boys from the Hairy Hooligan and Meathead Tribes, gathered on Thor'sday Thursday for the Final Dragon Initiation Test. This was to be a demonstration of a dragon's obedience and training. Killer took a dislike to Fireworm, a Monstrous Nightmare belonging to Snotlout, as she felt he was inferior to her. Ultimately, triggered by Toothless, all the boys' dragons dissolved into fighting. Killer and Fireworm fought each other the hardest, their disobedience causing all of the boys of both tribes to fail the test and be exiled.
Fighting the Green Death
The next morning, two Seadragonus Giganticus Maximus - the Green Death and the Purple Death - washed ashore on Berk. As part of a plan concocted by Hiccup, Killer and the Meathead dragons bombarded the Purple Death with feather bombs and called him names, leading him to the Green Death. Insulted, both of the large seadragons fought to the death.
Killer is a large male silver-colored Monstrous Nightmare. His wingspan is estimated to be about four feet.
- "Hiccup presumed he was Thuggory, Chief Mogadon the Meathead's son, because a silver-gray Monstrous Nightmare about three feet tall was perched on one of his shoulders. It was looking at Fireworm like a Rottweiler thinking evil thoughts."
- ―Book 1
Killer is describes as "a dragon with a very short temper". Indeed he cannot tolerate Fireworm very well. He says nothing, only growls, possibly a sign of grumpiness.