Demon slayer manga chapter - 143 explanation | BATTLE OF BROTHERS !

The chapter begins with a flashback of Shinobu-san. In her childhood, she believed that life was the most beautiful and happiest thing in the world. However, one day, a demon attacked her family, shattering that belief. She realized that their happiness was as fragile as a thin layer of glass. During that attack, she and her sister were rescued by the Stone Hashira. From that moment on, they promised to defeat all demons.
Back in the present, Shinobu thinks to herself, "Even if we're weak, even if we can't cut the demon's neck, if we can defeat one demon, we'll save ten people. If we defeat an Upper Moon, that'll be hundreds of people. It's not a matter of if I can or cannot. It's just something I have to do."
In the next panel, Tanjiro asks, "Shinobu-san, are you angry?" Shinobu responds, "That's right, Tanjiro-kun. I have been angry for a very, very long time now. My parents were killed, my sister was killed, all of my successors were killed except for Kanao. The same is true for those girls. If all their relatives hadn't been killed by demons, they would have been happy with their families now. It just drives me insane. Why isn't the poison working against this guy? Oh, come on."
Shinobu falls to the floor slowly. Suddenly, Douma hugs her and says gratefully, "That was great! You worked so hard. Are there any last words you want to say? Tell me, I'll listen to you." Shinobu, furious, replies, "Rot in hell."
Suddenly, Kanao bursts through the door, shocked, and screams, "Master!" Shinobu's sword falls to the floor as Kanao attacks Douma with her "Flower Breathing, Fourth Form: Safflower Robe." Douma quickly dodges the attack and says happily, "Oh man, that was really close. Don't slash me while I'm in the middle of absorbing, please. But man, it's just a really good night. I get to have one gorgeous meal after another."
Kanao is shocked and unsure of what to do next. In the following panel, we see Zenitsu facing a door. A voice comes from behind it, saying, "But people, for some reason or another, still try to convince themselves that it is one." Zenitsu responds deeply, "I know you're here. Come out already. You're just dawdling over there."
The door suddenly opens, and the voice continues, "That's not the right way to address me. I'm the older disciple here. You may have improved a bit, but you're still as dainty and seedy as ever. Long time no see, Zenitsu." The new Upper Moon Six appears, revealing himself. Zenitsu calmly says, "Kaigaku, now that you're a demon, you are no longer my senior."