House on Haunted Hill (1959)

Continuity mistake: Frederick picks up a bottle of champagne out of the ice bucket and shakes it a little. As he does this the foil and wire are still on top. He turns and aims the bottle at his wife, then tips it back and the foil has vanished as he now just removes the wire and cork.

Frederick Loren: The caretakers will leave at midnight, locking us in here until they come back in the morning. Once the door is locked, there's no way out. The windows have bars that a jail would be proud of, and the only door to the outside locks like vault. There's no electricity, no phone, no-one within miles, so no way to call for help.
Watson Pritchard: Like a coffin.

Question: (Spoiler alert) If Annabelle and Dr. Trent simply wanted to murder Frederick, what was the point of Dr. Trent attempting to dispose of Frederick's body in the acid? Why the elaborate scheme to have him accidentally killed by someone else if they were going to destroy his body in acid anyway? They could have simply hit him over the head and dumped him in the pit.