He most likely found out the leaf were responsible for his parents death, so he most likely wanted to take revenge
I wouldn't be surprised if Kishimoto forgot
You're wrong, it's Kishi's style to develop some characters more while he keeps some other hidden to become the real threat later. In my opinion what he did with Orochimaru is ingenious. He presented him now as an ally, he even wnet as far as to nullify him with IT, only for him to become dangerous once again later, a prime example of similar "indifference" was Spiral Zetsu. For many, he was completely forgotten. As for Kabuto, you don't see the whole image. Just think that that Tsukuyomi makes a sec pass like 3 days, who tells you that 2 hours in Izanami can't be much more than that, let alone that it won't be simply 2 hours.I think at this point it's safe to say that Kishi can't handle too many separate character developments very well. He tends to choose a few favorites, though those few change from time to time. He's forgotten about Orochimaru and the Sannin a long time ago...
EDIT: Orochimaru and Kabuto only came back recently as plot devices (to help Sasuke), but Kishi kept their characterization to an almost annoying minimum. Orochimaru's dead stare when asked about his motives, and Kabuto saying "I changed my mind in the last two hours because Izanami, so I guess I shouldn't have started a world war and killed thousands of people. That's how long it takes for mass murderers to change their minds, folks."
You're wrong, it's Kishi's style to develop some characters more while he keeps some other hidden to become the real threat later. In my opinion what he did with Orochimaru is ingenious. He presented him now as an ally, he even wnet as far as to nullify him with IT, only for him to become dangerous once again later, a prime example of similar "indifference" was Spiral Zetsu. For many, he was completely forgotten. As for Kabuto, you don't see the whole image. Just think that that Tsukuyomi makes a sec pass like 3 days, who tells you that 2 hours in Izanami can't be much more than that, let alone that it won't be simply 2 hours.
Well, I'll give you the Kabuto thing (I wish we'd seen more of his thought process/torment, though). But how is Orochimaru coming back, completely off-character as a plot device "ingenious"? You say that as if you had the foreknowledge of his eventual return after IT. If and ONLY IF he does return, then I'll concede you the point that Kishi didn't forget. But for now that's just speculation...
Spiral Zetsu didn't really come back as a rounded character. Same as Oro/Kabuto: just a plot device--in this case to keep the Kages/SA away from the main battlefield. We didn't learn more about SZ, mainly because there isn't anything to him, he's just a henchman, so I don't think he's a good example of Kishi handling a large cast well.
Right now, the only people being explored are the people in the battlefield. We got a small glimpse of some of the Kages and Konoha 11 during IT, but that was pretty quick. Like a "Hey, don't worry, I guess I remember these characters exist." That was the same chapter that we just saw Bee standing there, forgotten... lol he's been alive this whole time, just standing there...?