Hello, this is the reason why the owner didn't make a good transition. Let the old cat accept the new cat and get familiar with the smell. Wipe the kitten's body with a towel to absorb its smell, and then put the towel in the place where the old cat often walks. When the kitten is familiar with the quarantine area, cancel the quarantine observation. Old cats are not aggressive to kittens and can interact together. How to do this can refer to this article: How to make old cats and new cats coexist peacefully.