Joker (Earth-96)

"Police suspect that The Joker is behind the attack. The coroner’s office representative described the scene as the worst mass-casualty situation in the city’s history."

- Daily Planet article

The Joker was an attention-seeking reject from Gotham that attacked the Daily Planet with toxic gas, killing dozens of people.

Biography
The Joker, a Gotham criminal, felt that the Daily Planet in Metropolis didn't give him enough attention, so he paid them a visit to play what he called a "practical joke" on them. Deploying a noxious gas, he slaughtered at least 48 employees and injured dozens more. This despicable act of terror would take Clark Kent's friends and wife away from him in one fell swoop, putting him through more loss than what any mortal man could endure.

Legacy
What The Joker's massacre put Clark Kent through would go on to ensure he would be the Paragon of Truth when the Crisis of 2019 dawned upon the multiverse, although it was ultimately in vain as Lex Luthor used the Book of Destiny to usurp his position among the Paragons at the last second.