It's ironic because in Batman Beyond: Return of the Joker, the laughing gas was considered the tame way for Joker to kill someone. He kills one of the goons at the start of the movie when the goon argues with him. In the censored version of the film, the goon gets gassed to death with the grin on his face and everything. In the uncensored version, Joker just straight up shoots him in the chest.