who were responsible for the Mahabharata?

Directly and indirectly everyone has to be blamed .

Directly . the one who should be toatlly blamed are

Drupad and Shakuni- because they were the only people who wanted the destruction of kuru dynasty . and reason is known to everyone.

Now their pawns " Draupadi and Duryodhan .

We all know drupad gave birth to draupdi for the destruction of kuru family by marrying Arjuna. And Duryodhan was used by shakuni .he instigated him against pandavas ,especially yudhister .

Now these were direct causes . now the indirect reasons for these causes

.Drona was the reson for drupad and Bhisma was the reason for shakuni .

Now the direct signal for war.

Drupad made draupadi choose Arjun and shakuni made duryodhan choose karna.

Now on the basis of Arjuna ,pandavas never feared and were ready for the fight from the begining.

And after Karna , Duryodhan was hell sure for his victory . he actually chose the war because he had great trust on karna .

Generally we should not consider the indirect causes or signal . else there is a long list for them . like ..if pandu had not left his throne ,yudhister would have become successor easily . if kunti had not abondoned karna ,he would have besides pandavas and duryodhan would have never dare to fight against Arjuna and karna

So directly - indirectly these people are responsible for the war