You have a regurgative reaction to mistruths.
Benoit is speaking with Marta, who has an unfortunate reaction to telling lies. She vomits each time she is being less than truthful, which is an unfortunate trait for someone who is trying to come up with a false alibi and also pretend that she had nothing to do with a mysterious death. Benoit is also aware that Marta seems to be a decent person, and ironically this is why her reaction to lying is so severe. Marta's tendency to throw up when she lies adds a darkly comic element to the movie.
Marta Cabrera : "You're not much of a detective are you?"
Benoit Blanc: "Well to be fair, you make a pretty lousy murderer."
Each of the main characters has a flaw, and that is a lack of ability in their chosen role within the events that unfold on screen. Benoit is not a very good detective; he goes with his gut reactions, which are often not very accurate. He trusts dogs more than he trusts people, and he does not think that the evidence is actually pointing towards the perpetrators of the perceived crime. He is also quite defensive; he doesn't like to be criticized when he doesn't feel that he is doing that bad a job.
Marta does make a lousy murderer. She cannot lie, she can't keep her story straight and every time she lies she vomits violently. She is also blissfully unaware that she benefits in any way from Harlan's death. Finally, she is so busy feeling guilty about a mistake that she didn't make that she does not realize this guilt makes her singularly unlikely as the murderer.
