The butler did not do it, but he was (possibly unknowingly) a vital part of the murderer's Evil Plan.
The butler broke his master's leg.
The butler did it because he's actually an assassin that took the job to be Beneath Suspicion.
A butler is someone who, in a wealthy household, has a great deal of access both to the 'upstairs' and 'downstairs' areas, is usually a trusted and respected part of the household and, as a servant, may receive a lot of poor treatment. They tend to know the hidden secrets of the household and the various enmities and weaknesses of those within it that can be exploited, and know where various weapons, etc. are kept. They're usually organized, discreet, thoughtful, and methodical. All helpful things if you were planning to kill someone...
The master is not Nice To The Butler, and the butler knows that if he quits, he will not get a character and so will starve. Years of slights finally resulted in an outburst.
Every suspect except the butler had a hand in the murder.
The butler is the detective, or helps the detective find the real murderer.
The butler is the murder victim.
The butler is the prime suspect at the beginning, and is later found innocent.
Putting on a butler suit brainwashes the wearer to commit murder.
The victim doesn't even have a butler, there are no butlers anywhere around, and somehow the butler still did it.
Butlers learn their trade at butler camp where, among other things, they are taught how to get away with murder.
Zig Zagged: The butler did it, but he was under Mind Control at the time. And it later turns out that the one mind controlling the butler looked exactly like the butler. And then we find out that it was actually his Evil Twin, who was also a butler. But it was a conspiracy and the butler has a Memory Gambit to maintain deniability.
The household does not employ a butler, and there are no butlers anywhere in or near Troperville.
A butler is employed by the household or otherwise present, but is in no way related to the murder.
The butler had to be the killer, because the producer was aiming for target audience of people who never read mysteries before.
A person high up in the production once had a slow and incompetent butler, and writes the story to get revenge.
Invoked: "We should investigate the butler first. It's always the butler in the movies."
Exploited: Someone in need of a hitman goes to the butler, knowing he's good at killing people.
Defied: "We have to lock all the butlers up before they can kill!"
Discussed: "Unlike what you may read in detective stories, the butler is an unlikely suspect in any murder investigation of this sort."
Conversed: "These murder mysteries are too predictable. It seems like the butler is always the murderer."
Implied: The detective rules out all the guests one-by-one, but in the end he fails to find the real killer. The astute reader notices he never bothered to investigate the butler.
Deconstructed: The butler is a revolutionary serial killer, who purposely takes jobs as butlers to murder his rich masters. All the Unfortunate Implications of class warfare that this suggests are brought up and discussed.
Reconstructed: The butler did do it, not out of personal motivation, but for a large sum of money - and the rest of the story is about figuring out who paid him to murder his master.
Plotted A Good Waste: The butler did it, and the unfortunate implications of class warfare are touched upon as a result.
Played for Laughs: The Butler did it, but it took him three hundred and seventeen tries (and we're shown twelve), all of which his master escaped without realizing anything was happening (including the time when he walked up and shot his master, which the master passed off as "you could've hurt someone, mistaking that gun for a lighter.").
Played for Drama:
The butler did it, but is quite sympathetic, and the reasons he did it are explored in great detail.
Because he is no longer employed (having killed his master), the butler ends up homeless and/or is Driven to Suicide.
Untwisted: The butler is the nicest and most sympathetic of all the suspects, so the audience doesn't expect him to be the murderer. However, he unwittingly admits that he did it after the main suspect is arrested.