To what extent was Mao Zedong personally responsible for the Sino-Soviet split?
Mao was responsible for this split to some extent, but he was by no means the only cause of that split.
Mao can be held responsible for the split because part of the split was over interpretations of Marxist and Leninist theory. Mao, for example, felt that doctrine called for a more confrontational approach to the West than Khrushchev wanted. Of course, one could equally say that Khrushchev is at fault for disagreeing with Mao.
In other ways, the split cannot be blamed on Mao. In some ways, the split was all about national interests, which were not invented by Mao. The conflict over where the border between the two countries was went back centuries. The conflict, therefore, was caused partly by ideological differences and partly by issues that had more to do with national interests and power. Mao was not solely responsible for this conflict.