To a Chaos God, the worst case scenario is if an Everchosen succeeds but one of the others benefits more than they do. The second an Everchosen is going to deliver a success that benefits one of the Chaos Gods over the others, the other three will turn their full attention towards changing what success will look like, instead of on making success happen. In theory the reason Archaon was supposed to be such a threat was that he was aware of this dynamic and actively worked to keep the Chaos Gods balanced and therefore on task. In practice this turned him into something of a generic evil guy with zero personality, which IMO is probably a big part of why he failed so hard in Storm of Chaos - the results came from people sending in the results of actual games they played (or claimed to have played) and there just weren't many people excited for Archaon the Everfailure.
Malal was the product of the guys that created Judge Dredd and a bunch of other comic book antiheroes making a comic in the Warhammer universe, the core idea seemingly being a protagonist with Chaos Warrior aesthetics but killing bad guys. That protagonist's name was Kaleb Daark, which kind of says it all. But GW was still new to the copyright game back then and the writers of that comic ended up owned Kaleb, which left a question mark around whether they'd also own Malal, so Malal got excised from canon too. He gets brought up as some secret deep lore when he was just a weird incursion of the era's comic book memes into the Warhammer Fantasy ecosystem. There are more recent incarnations of the same general idea in Necoho, Chaos God of Atheism, and Zuvassin, Chaos God of Foiled Plans, but even they have only ever gotten brief and vague mentions since 1e. Chaos just doesn't need a secret saboteur to explain what keeps going wrong, the four main Chaos Gods are perfectly capable of sabotaging themselves and each other.
