Black Cat DQ's 17-21

17. Because he realizes the cat is now gone so he doesn't have to wake up to it every morning. He's now all alone and proud of what he's done.
18. He was saying he killed and got away with it and he still breaths as a free one until he is caught and imprisoned.
19. Because they don't notice that there is anything different with the wall and so he though they wouldn't never know.
20. It's showing how the screeching low noise progressed into a loud scream.
21. He's saying the corpse of his dead wife stood there staring at him and the cat was sitting there. He had put the cat in the wall along with his dead wife by accident.