In Germany each store that sells batteries has (has to have!) drop-boxes for old batteries.

Never throw batteries in the trash as 1) they contain toxic materials, and 2) (especially on a plane) even almost empty batteries can cause a short-circuit when connecting to each other or with metal in the trash, potentially resulting in heat and a fire.

Thus, the FA’s response might’ve been blunt and inappropriate to a customer, but your request was not a smart one …

And by the way: battery-recycling is a pretty big business, thus not 99% are sinners …