I agree when you say the book is her reality at the moment but to a man who in the brief moment you met makes one realise what a vile character he is, and also to the relentless survivalist character the protagonist is... Her decision to spare him on account of future guilt makes it confusing and rather unbelievable.
Well, in truth she seems like someone who would try to get rid of someone like that with violence. When she was still engaged to that douche, it seemed like she tried to kill him (I think it was within the archery or something). I think the reason why she told Adam to spare him was (probably) mainly because she didn't want Adam to be the one to kill him. And a little bit of guilt, I guess.
Kinda hate it when an MC decides to spare an enemy that tried to kill them and the enemy just tries to kill them again.( ̄∇ ̄")