The difference is, that Saphira had loving parents and was probably normal, before the fire incident.
She was an older teenager, when she was sent to the orphanage, while her sisters were much younger. After so many attempts to save them and getting her broken so many times by Sigmund, she snapped; with guilt eaten her everyday due how she couldn't protect and save her sisters.
I imagine she became so bitter and "insane" because of the hardships and conducted herself into harsh (mental) training, which lasting effects on her (like Bea from PKMS), willing to kill and destroy the ones, who have negative intentions towards her family (best example= killing the grunts in the lake or in the Meteor Base, in case the children were kidnapped).
As for Heather, her mother was killed when she was a baby and had only her father, Corey, as parental guidance, but his nihilistic behaviour and his ways had a very negative impact on her, making her very independent, but same as Saphira- the difference was the age- and isolated. But she had her past's actions reflection and had a great character development during the Ametrine arc.
It's been months, since I finished Ep18 and played the game, so I could forgot something