Someone buying this costume is basically dressing as a person who was RESPONSIBLE for many of the KINNOS that we say on TISHABAV. Knights were promised great rewards in heaven by priests for killing Jews.
This is somewhat incorrect. The Rhineland massacres took place at the hands of the People’s Crusade, which comprised mostly peasants and freed criminals who were easily whipped into a frenzy by the clergy due to various factors.
(That is not to say that the knights and titled nobility were not just as bad, and didn’t kill Jews on their own. Just that you seem to have mixed up your facts.)