Moral Flexibility
Cobra was somewhat slow and fragile, but its bites were lighting-fast, and its poison highly venomous. His hood and scales could also hypnotize his victims, compensating for its poor speed. Keeping it close to him and his dragon was the best way to take advantage of its faculties.
Last but not least, Chimera was the main fighter of their squad, even more ferocious than Charizard and Vulture. Despite his potential, his dragon was still young and could not compete with this highly versatile hybrid predator. It hasn't shown much until now, but the beast was strong and fast despite its mass, and its eight spider-like legs ending in clawed pads allowed it to operate on any terrain, no matter how rough it was. Will had seen him snapping in half a big tree with his mandibles a few minutes earlier and this scene had made a strong impression on him.