House on Haunted Hill (1959)

Factual error: As Frederick is explaining to his guests how to use their .45 automatics, he fires one round at a near-by vase, shattering it. However, it leaves no bullet hole in the wall directly behind the vase.

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.