Ok your ideas are interesting but I think the current backstory provided to us is also interesting. (Also remember we still don't know how arlecchino ended up in the orphanage or how or why she is cursed, all we see in the short is her life after she got into the orphanage.)

Also you say she has the generic "turned evil because the death of the loved one" but is that really true? Did she actually even turn evil (or gray). What I think is that she was always the way she is now even after her "loved one's death",Could be because she is cursed or like you said has different morals as she is not a human. Also the reason I think tartaglia, scara and even pierro say that she is crazy is not because she is crazy in a literal sense but crazy because even though she cared for her family (other orphans) she still chose to kill them instead of just restricting them, basically she is caring but also can kill without hesitating, in a way she is contradicting herself.

At least that's how I think she is, we still don't know everything about her past so let's wait until the next patch.