Between the theme, and some lingering NPE resentment from 1/2e, and the fact that Plague is written as a jerk, he's hard for people to like. Though I'm a particular fan of him.
The original Hamelin was devoured by Plague's vermin when he fell into the prison beneath the sewers. Hamelin's body is just a shape that Plague takes, formed of vermin, because it's "easy," he has changed the face over the years though.
Plague first tried to rebuild Nix as a vessel before Hamelin fell, but Nix's spirit was too loyal to Hamelin, which is why he still follows Plague despite Plague having no direct control of Nix.
None of the Tyrants like each other but there's particular resentment toward Plague because he tends towards plans that "steal" from other Tyrants rather than building himself, Siphoning off the Red Cage, weakening opponent's followers, stuff like that.
OG plague was an alchemist, but that term gets weird in Old Malifaux, just because of the godlike powers available then.