Black Mask

From Stephanie Brown Wiki

Tortured Stephanie with a power drill and then shot her, injuries which led to her 'death'.

BUT if it makes you feel better, he was killed by Catwoman. Twice.

There! That's much better!

(The thing is, we would need a thousand pages of Catwoman killing Black Mask to even begin to make up for even that one single panel of sexualized horror from War Games.)

(But this will do for now.)