He's single handedly killed over 4,000 innocent people personally, detonated at least three nukes and around 13 unsuccessful ones. He's renowned for his Joker Toxin which causes death by suffocation and leaves it's victims horribly disfigured. He's taken on the likes of the entire Justice League and won and is even feared by other villains. Trickster once said that when villains want to scare other villains, they don't tell ghost stories, they tell Joker stories.