Here is your justification for the amnesiac assassin character (no child in this justification): Faye Valentine.
The bottom line here is this: the amnesiac character type does not have to be an emotionless wretch. That's not how amnesia work. You lose your memory, but you don't lose the reflexive responses that is inherent to a human being. Further, even if an amnesiac was truly an empty container, that container does not, should not, stay empty. Socialization happens no matter what, and a personality is going to develop no matter what. Shows that do not allow that to happen to a character is just doing a real shitty job.
Back to Faye. For those that haven't, watch My Funny Valentine and get back to me. Faye is an amnesiac. She wakes from cold sleep in a Don Quixote state hundreds of years in the future. What does the show do with her character? Immediately saddle her with a burden: a whole shit ton of debt. That is her drive to grow as a character established early (early within the chronology of the in universe timeline), and it does wonders for her character. So I say bring on the horrible amnesiac characters that I can wave off and ignore until we get another Faye, at which point I am all on that.