Sometimes it's fine to have a villain who is just a force of nature and out-and-about bad guy. This is the kind of villain that can carry this type of event after all. Sure, Knull in that sense is kind of simple, but I don't think he needs more. Any kind of elaborate motivation would just ruin the point of him being an elder god who is a jerk and wants everyone dead. There is no need for more depth than that in this particular case.
Besides that, I think he is fine. Sure, he is certainly powerful, but he was not created to be a villain Venom could beat by himself in an alley. He was meant specifically for an event like this, and if he comes back it will be in equally high stuff too. Nothing wrong with that either.