These realms are very important to the demons as most of the time and usually don't have the power to defeat/kill enemies above them. Also there are rare geniuses of the demons that can kill enemies a sub realm or even kill a being a major realm above them. They are heaven defying geniuses that come every couple millions of years and at least one is born every millions of years to one race but the highest heaven is been so lovely that it gave 3 to the declining demons. They all died due to being naive and a simp for a Human woman.
Humans can kill across realms but it depends on what techniques they have.(some demon can learn techniques but they are rare.)