Cuz he gains nothing from their death, but she might have thought the people who took him had bad intentions themselves
Nah technically he does. Killing her family means his kingdom can now win against FL's kingdom. Before, just the presence of FL's father meant they didn't dare to attack FL's family.
But yeah, I think it's because she doesn't believe ML could be the type of person who would betray his saviors like that.
Also, if the ML did kill her family then there would be no need for him to marry the FL, and bring her and her brother over.
The story is amazing but honestly I wonder how fl never doubted that ml in some way was responsible for the massacre of her family
And also the brother is kinda annoying but I can't figure what about him annoys me