Never assume anything. It could be a song from 1/2 way into the series. It could be a tribute to Dr. Frankenstein. It could be a song about how Inori was immune to the Apocolypse virus, and her blood is a serum/ vaccine. I'm yours could imply she belongs to the entire human race, or those who need her. And you assume that she wrote the song herself. Guy might have written the lyrics for all we know. She does tend to do things as he tells her.
No one has posted anything to counter my argument that she does not exhibit much in the terms of self. So whether she is the most altruistic human alive, or just the best soldier out there is a matter of guessing right now. I'm leaning best soldier. I think she doesn't know enough about her own past to have formulated any design to living her own life, and making the choice to be altruistic.