So here's a bit of a odd theory I've had for a while what if Joker was a undercover cop? That would explain a lot of his different back stories and his various skills and talents. My main theory is he was the one good honest cop and genuinely believed in a better Gotham but then because of Gordon and Batman (like there actions could have affected his investigation) his family was put in danger his cover got blown and finally he fell into the chemicals. I just think this really fits him and really adds to why he hates Batman and has tormented Jim Gordon at the same time I like the idea of Jim and Batman's earlier days having unintentional collateral damage.