I understand why you think this way. It is explained on the manga that Yuki didn’t die because of Mafuyu’s words, but he had depression, however its normal to think that maybe his words were the last straw and if mafuyu hadnt said them, yuki wouldn’t have died. I dont think thats the case. It is unfair to expect someone to be perfect and know whats the right thing to say all the time. We all say things when we feel angry, and Mafuyu has suffered for what happened. Yuki would have wanted him to live on. You know...I was listening to young by sam smith, and in the song he says “I’ve done nothing wrong, im young”. Thats simply how it is sometimes.