If Belos believes that witches are monsters Joker will show him what a true monster looks like in the form of a smile and a sick joke. Plus Joker hates British people and Nazis.

Belos is a genocidal maniac and a puritan and Joker is psychopathic gangster, a schizophrenic clown, and an anarchist philosopher. While Belos has a reason to kill because of his delusional beliefs Joker, he will kill anyone for any reason or no reason at all.

Joker lured people outside with millions of dollars, only to poison and kill them with gas, for seemingly no reason.