Tamahome lived and breathed for his family. And no matter how much he loved Miaka, or even the fact that his father was now cured he could never abandon them completely. So for Tamahome to be able to give it his all and want to be reborn in Miaka's world he had to have no strong ties holding him to the universe of the four gods. So his family had to be destroyed.  So what if  Tamahome’s family had not been murdered by Suboshi? Would he still have made the same decision? Naturally, Miaka is very important to Tamahome, but if his family had stayed alive, he would have had to chose between Miaka and his family.

Tamhome wanted Miaka to live with him in the Kounan empire but after he saw Miaka's world he realized that she had family and a life that she could not easily give up. Also we are told a bunch of times that it's impossible for the miko to remain in the book after she has summoned her god. So Tamahome would have had to chose between his family and Miaka. I am certain that Tamahome would not be able to make such a decision…but if such a decision came his way, which would he chose?

He could never leave his family if he thought they would be in danger and if his father was still sick. Tasuki and Chichiri would have offered to look after them, but would that have been enough for Tamahome to give them up for his own true love. Who knows? and this is why Tamahome's family had to die.