If reliving the lives of his ancestors taught Desmond anything, it was to fight for what is most important to you.

For Desmond his world was small--he did personally learn of strength through wisdom, of finding deceit in a beloved master's teachings, he did not personally know of true liberty, of love, and of time, when Ezio's family was cut down before him and he was raked with vengeance for years on end, and Desmond did not know of freedom fought for in times of uncertainty and through shades of grey when Connor both won the war, but lost everything he had cared for.

For Desmond, he tries to live up to the saying he learned in the beginning, that he heard over and over, Nothing is true, especially when in Desmond's heart it feels wrong, and everything is permitted--especially when one has to live with the consequences, be it simply life, or of death.