From the letter, Elizabeth learned a bit about the nature of the Burning Heaven Sect and how they had no interest in expanding outside of the Ishaan Kingdom.
The sect master explained how they were a 'good' sect and did not see eye to eye with an 'evil' sect like the Bloodfiend Sect that treats human lives like dirt. However, they were not as powerful as the Bloodfiend Sect and could not do anything to them until now.
In a few words, the sect master had differentiated the two sects between good and evil.
The Bloodfiend Sect would have been uprooted eventually. Still, the Burning Heaven Sect intervened to establish friendly relations. They believed they could get along well despite there being little to no interaction between their sides due to the great distance between them.