"I killed my best friend because he stole my fiancee."
As how Chichiri told it for the first time
"I killed my best friend because he stole my fiancee. "
"So you should not give up on saving her, Miaka. Your friend."
In the perspective of the anime
Chichiri just caught Hiko, (his childhood-frined) and Kouran , (his fiancee, who had also been his childhood friend) kissing after he announced his and Kouran's engagement. Chichiri was furious at this and was enraged. Hiko tells Kouran that it hurted to love her, and after this, a flood followed, and claimed Chichiri's family, loved ones, and his best friend's and his fiancee's life.
In the perspective of the manga ( The true story)
Chichiri announced his and Kouran's engagement to Hiko first, telling him that he would like Hiko to be the first person to know. Chichiri was hiding on a tree, holding a kind-of gift for him to give to Kouran and was wearing a big smile. But when he turned to look, Hiko and Kouran were already kissing,
and Kouran apologized and said that she can't be with him like he planned.
A flood followed and Chichiri held a knife over the neck of Hiko on a small rock-cliff and Chichiri told Hiko to give Kouran back. Suddenly Hiko slipped of the rock cliff and fell to the powerful waves and water currents the flood produced, and Chichiri held on to Hiko's hand. Chichiri let go, and another moment he planned to save him, but a tree crashed on him and gouged out his eye.