The Dunyain where right. Even if the world works differently than they thought, it doesn't matter.
Even if they can't reach the absolute (or if the absolute is very far away) they are the only ones on the right path . Imagine if Kellhus where 2x, 5x or 10x "closer" to the ideal, surely he would have succeeded.
The series is an argument between intellect and irrationality (feelings) with the scales going from one to the other. For 2000 years the Dunyain where pursuing Intellect, so our main character ( Kellhus ) starts out at that end. The moment he leaves Ishual he is broken. I think few people understand how significant this is for his kind. Moengus didn't suffer this fate. After that he follows the shortest in some ways, but in others he is deceiving himself.. He loves Serwe, Esmenet,Cnaiur, Akkamian and even Proyas, the worst sin however is that he loves himself.
Every mistake Kellhus makes is because of love...Moe is his last test. Instead of talking to him (someone more experienced both with the world and the Logos) and figuring out the shortest path, he gives some bullshit justification and he commits the murder. He is afraid that the shortest path might require his own death, he is afraid of a truth that inconveniences him. He chooses self deception.
Now there is nobody who sees as "deep" as him, nobody who can see his darkness. This way Kellhus becomes just as anyone else in this world. Talking and debating with your equals and betters is the foundation of rationality, of the Logos. The Dunyain accomplished their great feats with fanatical cooperation, every generation "building" the next. Now the only person he can talk to is himself.
What comes before determines what comes after. Because of these mistakes and a few others, he goes back after Esmenet. Even then, he still has a choice. Letting Kelmomas live was the culmination of his irrationality, a process that started more than 20 years before.
Kellhus was the harbinger and everything that happened was his fault. A "stronger" Dunyain would have made the same choices but without the mistakes. He had plenty of opportunities but again and again he didn't stay true to the Logos...
The world will burn because Kellhus didn't follow the shortest path, the world will burn because Kellhus loved.