Cerulean was broken by the HEROES during her magical girl training. She basically is a child soldier, and child soldiers are something that most actual soldiers seriously consider mercy killing on sight, as the strain of violence on a young mind usually drives it to not only be broken and tormened by past experiences (
vicious PTSD), but also to be horribly violent and lacking empathy. Recovery is very unlikely - most former child soldiers are either criminals, homeless, addicts or a combination of the above. The lucky ones can integrate back into society well enough to be adopted, but even then they usually maintain close ties to psychiatric institutions for decades.
Ceruean has repeatedly shown stunning anger management problems, irrational fears of losing her surrogate family and a complete lack of empathy to those she considers her enemies. I have all the reasons to believe that all those traits are induced in her by her training.
Here is what I believe she is doing:
http://en.wikipedia.org/wiki/Acting_out - it's a defence mechanism that allows her to avoid painful memories of violence being directed towards her by being violent towards others.
She may not even love us, actually:
http://en.wikipedia.org/wiki/Undoing_(psychology). It is not unreasonable to assume that Cerulean irrationally hates us and Chemista in the same way she irrationally hates most people, but due to her fear of losing us as a family, she represses those hateful thoughts with saccharine-like affection.
Whatever the cause, Cerulean is a deeply troubled individual that is dangerous to those around her. I cannot believe I am saying that, but at this point I am seriously considering humanely euthanising her.