Focusing on the theme of suspicion and fear:
* The Web Tightens
* Shadows of Doubt
* Whispers in the Dark
* The First Accusation
* The Price of Trust
Reflecting the decreasing number of guests:
* Five Remain
* The Thinning Ranks
* The Circle Narrows
* One Less
* The Count Down Begins
Referencing the murder weapon:
* The Grip of Death
* The Red Herring
* A Tool of Fate
* The Silent Witness
More abstract, capturing the sense of dread:
* The Unseen Hand
* The Island of Secrets
* The Unraveling Truth
* The Price of Innocence
Ultimately, the best title will depend on the specific tone and focus you want to create for the chapter.
For example, if you want to emphasize the growing paranoia and suspicion, a title like "Shadows of Doubt" would be appropriate. If you want to highlight the dwindling number of guests, "Five Remain" would be a good choice.