Ever so often, Casters (magic users and the protectors of towns against demon attacks) have to make tough decisions to save as many people as possible. Like the trolley problem; one town needs to be sacrificed to ensure the safety of others.
Karson's town was abandoned by the Casters that were supposed to protect it. It was a devastating loss of lives. Lets say that my dear antagonist developed quite some hatred towards magic users.
And it suits snowflake because it was snowing and he felt very alone and very fragile.