This is evidence of how he killed Mr. Adams. He got caught with rat poisoning on his hands and that's how Mr. Adams died because of the poisoning. People at the party noticed Mr. Adams was gone for 25 minutes, so they all went into the kitchen and found Mr. Bennet. There was poison on his hands, Mr. Adams was on the floor, passed out dead.
Before the party started, Mr. Swanson saw and heard Mr. Bennet and Mr. Adam start arguing