"I've killed for you. Who else can say that?
Amy tells Nick explicitly that she's killed for him. In her mind, she's done something that no one else has or will ever do, as if it is the definition of the price she's willing to pay and one that Nick will accept as being a worthy reason to stay with her. Reality is that she killed in order to manipulate Nick into doing what she wants.
"You ever hear the expression the simplest answer is often the correct one?"
Gilpin states this as if he knows the answer to solving Amy's disappearance lies in Nick. He's oversimplifying a complicated issue that on the surface appears to be a binary solution, when in fact its gravely twisted.
"Nick Dunne took my pride and my dignity and my hope and my money. He took and took from me until I no longer existed. That's murder."
Amy clarifies her point of view with Nick in this statement. She tells us that he has murdered her through his actions towards her year after year in a way that has taken everything from her. Thus, even though she's alive, she was killed in a specific way by Nick.
