Disclaimer: I don't own Lost, nor do I make any money from writing this.
One would think that when one such as Shannon breaks, she becomes small, helpless, vulnerable. And, as such, that she becomes safe to approach. And sometimes this is true.
But sometimes Shannon breaks like glass. Shattered, but still keen and beautiful and dangerous. And, though fragile, still sharp enough to cut.
Shannon broke when Boone died (when he was murdered, no accident no matter what they say). She felt it, some part of her splintering into so many bright pieces. Everyone else noticed it as well, and most were perceptive enough to stay away.
Most, that is, except the one she wanted to see the least. He sought her out and sat beside her, even trying to act like he knew how she was feeling. She let him talk, even if inside she was screaming, Get the hell away from me! You've got blood on your shirt!
Likely he went away thinking her brittle and weak, too much so to do him any harm. It is there that he underestimated her for the last time.
He must have forgotten that broken glass is also brittle, but still sharp enough to kill.