Integrity measures the current value of your word-of-honor as a faction leader. If you keep your promises and do not betray your friends, your integrity will remain high. Attacking factions you have pledged Blood Truce or signed Treaties with will rapidly lower your integrity rating, and other faction leaders will no longer be eager to make such agreements with you.
Note that there is no dishonor in attacking a sworn enemy with whom you have not pledged Blood Truce; only breaking an agreement can lower your integrity.