I'm behind and maybe someone has already responded but I'll give it my 2 cents.
I suspect Kelsey was an empath and did not suffer from low self esteem. I've seen this happen in my own family. Narcs prey on empaths (as well as some who may suffer with low self-esteem). The empath's acute sense of empathy and a heart full of love, the type of love they have known, allows them to make all kinds of excuses for the narc. He's had a hard life, his mother's a "B", his sister is a "B", he didn't have a father active in his life, but I can make it better. My love can help him, change him. Well, sometimes you just can't fix that kind of "broken." When it doesn't work, they just try harder and harder because they continue to hope they will see that wonderful guy they met, who was just hiding behind a mask and his "love bombing", and when they see THAT man again, if only for a moment, a day, maybe even a week, they think their love is making a difference. But it is always short lived. These broken monsters CAN'T be fixed. Thankfully, not all narcs are murdering narcs. Unfortunately, KB met up with the worst of the worst who had found another narc to help him in his sick plan.
KB didn't suffer from low self esteem. At least not before PF got her in his evil snare. JMHO