Trust and Innocence
Riza has every reason not to trust anyone. She joined the military in order to help people, but instead found herself killing them. Throughout her time in Ishbal she saw the worst side of humanity and her naivety about the world, and her own conviction, were both lost. She was quite prepared to die after the war for what she had done, but was ready to acknowledge that her death would not grant her redemption. Her life, on the other hand, could still prove to be of some worth to the world.
Her faith in Roy can be traced back before either of them joined the military. Although her past is a complex puzzle with more than a few pieces missing, what we are able to glean has shown us that Riza's trust in Roy was to the extent that she placed the most deadly of knowledge into his hands - the power of the Flame Alchemist. With this Roy was able to take the state alchemist exam and become a "human weapon" for the military. She believed in his ideals, his will to help people and that he would use her father's knowledge for the greater good.
"I was the one who believed in you and entrusted my father's research to you. I was also the one who choose to go into the military academy wishing for the happiness of the people. Even if that had undesired results, I cannot run from the facts."
She never blames Roy for his destructive power. It was her decision to entrust to him and therefore the responsibility is hers alone. This is a surprising conclusion, but just reaffirms the forgiving and compassionate personality that Riza holds.
It is true that after seeing the war her faith in people, and herself, is shaken - but her faith in Roy is not. Both of them are confused - having seemingly strayed from the path they had originally set off on. But their is no blame there - had she not similarly done the same thing? Within Roy she found kinship; a mind that was like her own, affected by the loss of a life combined with a yearning to end the suffering of the living. When Riza is lost, Roy once again offers her a kind of salvation - to aid him in achieving his dream, his goal of helping the people of his country to avoid war once and for all.
"I have a favour to ask of you, Mr Mustang. Please burn and crush my backl. At the very least eve if I can't atone, so that I may never give birth to another flame alchemist! So that the secrets on this back can't be used."
At the end of the war she asks him to burn the knowledge off her back so that it can no longer be used to harm anyone. A symbol of contrition, a farewell to her loss of her innocence and her faith that such power in man's hand. But loss of innocence or no, her trust in Roy and her friends is unwavering. Despite the past and the suffering she has seen, Riza continues to love and to believe in those around her.
