As Wadsworth explained how the murders were committed, the killer did their best to keep up a poker face. It was possible that he was bluffing, that Wadsworth had no idea who the killer was and was trying to trick the murderer into giving themself away. If that was the case this person was not going to help Wadsworth, but feared it was already too late.
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Wadsworth: This was not a well-planned series of murders, but I must admit it was clever. Few could have planned all these murders so quickly, yet left no witnesses. With all of us here, there would be enough reasonable doubt for any of us. Only one person here is that intelligent, professor Plum.
Plum: I'm flatterred, but I'm not the murderer. And I believe you're trying to cast suspicion on me, to take suspicion off yourself.
Wadsworth: You made only one mistake. You kept the gun. Gentlemen, turn out your pockets. Ladies, empty your purses. Whoever has the gun shot the girl.
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Professor Plum took out the gun, thus exposing himself as the murderer.
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White: Why kill all of them?
Plum: It was not my intention, at first. I wanted to kill mr Body. Then I found the evidence he had on all of us, and destroyed it. I soon discovered that Wadsworth brought all of Body's informants here. You wanted all of us to murder them.
Wadsworth: Yes, and you did.
Mustard: So, now what?
Plum: There is no real evidence against me. If you were to all reveal my confession, I could just as easily point the finger at any of you. Not to mention all I know of you people. Imagine how much trouble all of you would be in if the whole truth came out. As for me, I work for the United Nations. You don't know what kind of people they have at the U.N., I might go up in their estimations. But the police would arrest me for murder, and I'd have a lot of company in prison, all of you. I'd suggest we stack the bodies in the cellar, leave quietly one at a time, and pretend none of this ever happened. If you go to the police, you will all end up in prison. And that includes Wadsworth, from a legal perspective he's guilty of reckless endangerment, or murder. I hope all of you keep that in mind as you make your decisions.