I agree, auggghhh! But wasn`t it fun?

It's not supposed to make sense by the end. It's all a big joke on mystery films or novels with convoluted twists and explanations to arrive to their conclusion.
It's a spoof, it's not a real mystery (it doesn't make any sense as such).

Eileen Brennan (July 28, 2013)just to be complete.
We interrupt this program to annoy you and make things generally irritating.

When you think about it. the 1st half seem.. so promising.. like a well executed script then.. boom, evolues into a convaluted, confused mush. Thinking it could be made clear at end and then that hysterical, annoying laughter at end is suppose to be interesting.. More like a carnival horror ride dummy
It was the scenes and the actors that made the movie..
Like Wang said.. 'Kill weekend'.
I sleep now.

Eileen Brennan

Tess Skeffington had some information that she presented to Sam Diamond near the beginning of the movie, when they were driving up to the house. (Admittedly, the information was provided to her by Lionel Twain, but it does offer some possible insight into what was really going on, and it might be a reason why Truman Capote is Lionel Twain)
Tess said that Lionel's parents were Catholic and Jewish (I forget which was which), marriage lasted only two hours, and when Sam asked, "Were there any children?" Tess said, "A girl." Lionel Twain was never Lionel. Or at least Lionel Twain was never a man. Orwell, you see where I'm going with this.
I'm still in the middle of rewatching this movie, it's been awhile since I have seen the full thing, butLionel or Rita or whatever her name is a master of disguise and disinformation, I agree with what others have said that Lionel/Rita played all of the extras, the butler, Yetta, the accountant, and Lionel.
The scenes where Yetta and the butler interacted with each other only happened in the kitchen, and were merely comic relief for the audience, and they never really interacted with each other. The butler was talking to nothingness, Yetta was there but she couldn't hear, and only the butler ever went out and interacted with the guests. What happened 'backstage' in the kitchen may never have really happened. Dummy butler and maid, certainly, but Lionel/Rita did inhabit them, I believe. Two of everything. Dummy Lionel got stabbed.
As long as there is space, I'm going to stare off into it.

You can now add:
Peter Falk (June 23, 2011)
to the list.

Who really died?
So far:
Peter Sellers (July 7, 1980)
David Niven (July 29, 1983)
Estelle Winwood (June 20, 1984 (at the age of 102!))
Truman Capote (August 25, 1984)
Elsa Lanchester (December 26, 1986)
James Coco (February 25, 1987)
Nancy Walker (March 25, 1992)
Alec Guinness (August 5, 2000)

I have thought about this long and hard. During the last scene in the mansion, it is all made clear. Everyone, but Sam Diamond, had it correct, to some extent. It is all the same person. The lawyer, the butler, the daughter, the accountant. The person admitted to be that person, but not to being Sam Diamond. If you think about it, everyone, but Sam Diamond, was correct, they just did not know who it truly was.

yea i was wondering where the butler went. i agree that they probably let him off for the night later on.

Probably Twain's former maid who decided to take over his fortune.

I absolutely agree, no murder actually took place, except for the good weekend of course. There was basically two of everything in that house, except for the guests, and as said before it was mentioned that Lionel Twain had died 5 years ago. So his body was obviously a dummy. Yetta had been playing as Twain the entire time. As Twain in the dining room, she said that the person killed was also the murderer. As I asked in another board i just posted. Does anyone know who Yetta really was?

Did a murder take place ?
Yes -
"Killed good weekend." - Insp.Sidney Wang.

Though there must have been at least a real butler at some time, because he speaks to the guests, and to the (fake) maid.
Laura Ess

I don't think any murder took place in the movie. But, I think the main suspect and host was Yetta. She was posing as Lionel Twain all along (notice that Yetta and Lionel Twain were never seen in a room at the same time). In the beginning, you see the butler talking to Lionel Twain, who was writing out the invitations. If you look closely, Twain's hands were feminine, meaning that they must of belonged to Yetta. If you put all the detectives' theories together, you can sort of figure it out: Miss Marbles said that Lionel Twain died five years ago; Mr. Charleston said that everything in the party was arranged (meaning the butler was hired for that one night); Mr. Wang said that the butler's body he examined was made of rubber, so the real butler must of been let go for the night. But, none of the detectives correctly guessed who the murderer was, so, Mr. Perrier's theory that the murderer was Lionel Twain's daughter Rita was incorrect. All the detectives left the night thinking that Lionel Twain planned the whole thing, but it was revealed at the end that he was Yetta in disguised all along. So, the detectives were fooled twice. Twain's dead body in the dining room was either a mannequin or Yetta in disguise. The mannequin of Yetta was made to throw off the guests. So, it was just one big game of whodunnit, but in realty, no murders took place.

This message has been deleted.

Lionel Twain died. He was outsmarted by his daughter Rita (who likes to be called Irene). She created a manequin butler and maid (basing the look of the maid on herself). She used these manequins to fool the detectives, but she couldn't make them see or speak. So
Who really died? - Lionel Twain (Truman Capote)
Who is who? - The woman at the end (the killer) is Lionel Twain's daughter Rita (Nancy Walker) NOT the maid.
Hope this helps.