I think people are forgetting that for our FL that the number of people who have genuinely been kind to her is very small—Shen Run/Yan Rui, her eldest and second brothers, eldest sister, and her two friends, her mother, grandma chen...many of whom have died or suffered due to schemes of others. She is of course a gaurded person who doesn't trust anyone easily.

She is surrounded by schemers who will stoop to any level, taking lives being the least of it.

She is also grieving the Eldest brother's death, mind you, this person went against his mother to help FL clear her mother's name.

So when Shen Run—the person who took an oath with her to seek revenge, someone who probably understands her much more than anyone, having shared the traumatic experience—now sets his own revenge aside and says he'll protect the very person responsible for it all, of course she feels abandoned and lashes out. To her, it must feel like the one person who was supposed to stand beside her, who repeatedly promised to do so, has suddenly apparently chosen the other side.

Anyone would feel betrayed.

I actually think FL lashing out at Shen Run shows just how much she cares about him and trusts him. She is usually so cold and poker-faced around everyone else, rarely allowing people to see what she is really feeling and thinking. But with Shen Run, all that restraint disappears and she lets him see her hurt, anger and sense of betrayal.