If Joker were to flip that'd be pretty cool. He could still kill people, but only bad guys, kinda like Venom/Brock. Or maybe Atrocitus, kinda like back in Brightest Day.
He'd be like...Superman...but only with the mentality of Frank Castle xD.
Like he saves a schoolbus full of children, does a line of K-Nite off one of their notebooks, scoffs at them and demands they stay in school and flies off immediately towards Lex Luthor, through the building just to clothesline his head off before Lex even has time to notice, just to spear himself right through the other side of the building and burrow through the back of an unsuspecting Metallo only to emerge from his chest cavity, thus killing him (again) and boosting his powers even more so and then while continuing that breakneck pace, swinging over to Gotham and frying The Joker who he overheard talking smack, along with his entire hideout with heat vision before elbow dropping Scarecrow who was trying to flee the location, immediately killing him while also leaving a nice crater so when it rains it will fill up with water an the Street Rats can drink fresh rain water until their heart's content (it's really slummy in this part of Gotham) then going to clean up a few more of the Dark Knight rogues gallery, before taking a nice break to cut up some more K-Nite for later on.
Like just think of the street level villains who would be wiped off the map xD.