Ok so the original story was posted on AO3 and i was so surprised how popular it got there i decided to publish it here so enjoy!
It had been 3 months since the final battle against Muzan, Tanjirou still didn't have much usage in his left arm and was blind in his right eye, Inosuke still had bandaging on his chest and arms from various cuts and bruises and Zenitsu still had bandaging on his chest and his right shoulder was still currently was recovering from the sixth upper moons lightning attack the cracks were getting smaller but any over strenuous use could reopen them as well as his legs that had just recovered from having cracks in his bone.
Currently Inosuke, Zenitsu Tanjrou and the former pillar Giyu, who now only had one arm, were sitting out in the garden talking ironically about the deceased pillars. "Wow did he really?" Tanjirou said.
"Yup walked straight into a tree after slashing its head off." Giyu said a small smile on his face. "Uzui found it hilarious and of course he just walked it off like it was nothing."
"Wow Muichiro could be scatter brained." Zenitsu said before saying panicked "Sorry I didn't mean to insult...!"
"Its fine even Muichiro found it funny later on." Giyu said.
"You know I always wondered about those kids Himejima-San looked after..." Tanjirou said.
"What kids?" Inosuke said mask up from his face currently having a mouth full of onigiri to Zentisu's disgust.
"The kids he watched over awhile back." Tanjirou explained giving a short explanation of what happened. "He said they betrayed him but I don't think that was the case, the only one I think that betrayed him was the one that led the demons to them. I don't think the others were trying to do so..."
"He told me about that" Giyu said in thought "...I remember the boy who took the protection away's name I think it was Kaigaku..." suddenly Zenitsu stood up.
"What?" He whispered and the three other boys looked at him confused by the unexpected behavior. "Kaigaku betrayed Himejima-san too?"
'Too?'Tanjirou wondered before seeing Zenitsu walk away.
"Ganitsu?" Inosuke said standing up pulling his pig mask on and he and Tanjirou trailed after him.
He moved much faster then expected since they lost sight of him when they arrived at the path that led up the mountain and had to use there enhanced senses to find him. When they did about 15 minutes after they lost track of him they were utterly shocked by the sight before them.
Zenitsu was standing by a tree that looked halfway destroyed, his knuckles now blood soaked. His back to them but a sort of rage they never had felt from the yellow haired teen radiated off of him. With a cry of fury he let out a punch at the tree causing more splinters to appear in the wood. Then he let out a sideways kick, something he shouldn't be doing since his legs were still under recovery. "Zenitsu!" Tanjirou called putting a hand on his arm.
He was ignored as the teen gave out another kick the tree was barely staying standing only one blow from falling over. "Zenitsu!" Tanjirou called again grabbing his arm trying to pull him back.
Zenitsu pulled away and punched the tree again. More blood pouring from his fists. "Zenitsu stop!" He said grabbing hold of the boy but was rewarded to an elbow to the gut causing him to cough and stagger back. Zenistu let out another kick and the tree let out a creaking sound and fell to the ground letting out a large crash that shook the clearing.
The boy turned and both Tanjirou and Inosuke were shocked by the pure rage in his features. He didn't seem to notice them as he went to express his anger on another object, a large boulder in the clearing letting out a growl in rage as he punched it blood from his hand splattering on the rock. "Didn't you hear Gantaro?! He said stop!" Inosuke shouted grabbing him by the arm and flipping him to the ground.
But Zenitsu countered with a kick to the gut from his position on the ground causing the boar masked teen to crash back due to the boys still strong legs.
Tanjirou took chance of Zenitsu's position on the ground and tackled him pinning his legs to the ground with his own knees struggling to get a hold of his fists which he punched out shouting. "Let me go!"
Tanjirou struggled only having one good arm and aching from his injuries managing to get a bruise on the cheek for his efforts but Inosuke came back over and with surprising gentleness from the hot tempered teen grabbed his wrists and pinned them down so he couldn't punch out.
"Damn it!" Zenitsu howled struggling for a few minutes longer until realizing he wasn't getting free and slammed his head back on the ground. "Damn!" He cried out tears coming from his eyes, these weren't his usual crybaby overreacting fearful tears, they were of true anguish.
"Zenitsu what's wrong with you?!" Tanjirou demanded having never seen the boy like this. The closest he has when he saw the boy sitting on the rock during there training days but instead of being suppressed anger it was unleashed into wanting to destroy everything around him to unleash it.
"Kaigaku!" He shouted.
"Gaikaku?" Inosuke said no one bothering to correct him.
"That bastard that fucking bastard! He didn't just betray gramps he betrayed Himejima-San and those kids too!?" He howled. "He's scum he's garbage he's worst then that!" Bitter tears running down his cheeks.
"Betrayed...you know him?!" Tanjirou asked.
"He was my senpai...he learned under the same master as I did...I respected him! But then he betrayed gramps and turned himself into a upper six moons demon! Gramps commuted seppuku and spent hours suffering cause noone was there to either finish him off or help him and died because of that piece of shit!" Zenitsu cried yet again hitting his head back against the ground again.
"He didn't even regret it in the end! I killed him! I respected him and he…betrayed...us….and" Zenitsu sobbed bitterly struggling for breath his anger finally turning into agony.
Tanjirou carefully sat up and Inosuke let go of his hands and he immediately cling to Tanjirou and cried his body trembling with gasps of air as he sobbed. All the pain and the sorrow he experienced Inosuke awkwardly patting his back as they watched there friend mourn not only a father figure but someone he thought of as an elder brother. Not only losing him physically but losing the trust he had instilled in him.
They didn't know how long they sat there, long enough for the sun's position to shift position in the sky, long enough that blood stained Zenitsu's outfit at the shoulder showing some of the cracks reopened. Long enough that when he finally calmed down he merely slumped again Tanjiro too exhausted to cry. "Aoi's going to kill me." He finally muttered.
"Yeah probably." Tanjiro said not denying it.
Inosuke helped them both onto their feet, Zenitsu stumbling causing Tanjiro to wrap his good arm around him. They headed down the mountain to face the consequences of Aoi's wrath.