It seems as if half the characters in How to Train Your Dragon 2 are amputees of some sort… enough that alongside the positive portrayals of Gobber, Hiccup, and Toothless, we also get a portrayal of the ol’ Amputee Rage trope, in which the amputee is filled with an all-consuming hatred for the person responsible for the loss of their limb, and self-loathing of their altered body. In this case, Drago Bludvist presumably lost his left arm and his entire family in a dragon attack on his village as a child. When he later came across a lone dragon, he abused it instead of training it properly, and began his campaign to subjugate both dragons and people. His heavy fireproof cloak of dragonskin is strategically placed to hide his prosthetic arm, in stark contrast to the numerous villagers of Berk with missing limbs.
Stoick describes Drago as insane, too; this is said of many supervillains both on-screen and off, with little regard as to whether they have a diagnosable mental illness or not. The tendency to think of violent or evil people as inherently mentally ill because their actions are incomprehensible has had the unfortunate effect of stereotyping all mentally ill people as dangerous. Film and news media focus on occasional lurid incidents, exaggerating the link between mental illness and violence in the minds of the general population. In fact, the mentally ill are more likely to be victims of violent crime at the hands of the sane, and suffer employment and housing discrimination as a result.