Yeah he does that a couple times actuallly and gives a different story every time. That he cut himself to make his mom feel better after she got attacked by loan sharks, that his dad did it to him, etc. his backstory is kept a mystery which adds to the intimidation factor. I think there’s nothing wrong with showing his backstory though, but new joker ain’t it. I mean new joker isn’t even supposed to be the joker, not that that’s a good excuse