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.)
Stephanie showed some fear and trepidation when faced with the idea of a new Black Mask in her own series, but she eacted as part of Batman's network to take him down. She kicked one of the villain's soldiers in the face. (Batman (697)