So you've got your victim, sleuth, suspects. You need your murderer. Maybe you've known all along who you were planning to tag as the perpetrator--maybe even before you knew who your victim was.
If that's the case, you may need to double-check your manuscript. If you've consciously placed all your clues to lead to your killer, your puzzle could be too easy for the reader to figure out. The times that I've solved the mystery (or had a pretty strong suspicion I've solved it) before the book's half-way mark, I was pretty disappointed.
To avoid pointing too obvious a finger at your book's murderer, consider the possibilities:
If all the suspects have motive, means, and opportunity (and they all should), then devise an alternate ending involving at least one (if not all) of your suspects as the perp. You may discover, in doing so, that you like another ending better than the one you'd originally planned. Remember The Mystery of Edwin Drood? Charles Dickens died before penning the ending of his novel. For the Broadway version, the audience votes on the identity of the murderer. The playwright devised alternate endings for each possible scenario. This, at least, will ensure that each of your suspects has a good, solid reason and means to kill the victim. If you find that isn't so, go back to the drawing board and beef up your motives.
Want to add a layer of complexity to your puzzle? Lead your readers in the wrong direction with red herrings. With your sketched out alternate endings there will also be clues that lead to that suspect's guilt. By placing clues that point your reader to a different murderer, you're using red herrings. Be fair, though, and don't lead the readers in the wrong direction for too long.
Have your suspects muddy the waters. If each of your suspects has his or her own agenda to promote (covering up a secret in their past? Blackmailing another suspect? Hiding an illicit relationship?) then they will be busily concocting lies throughout their interviews with the sleuth.
Expose a suspect's alibi as false. Maybe a character was completely passed over as a suspect because of his airtight alibi. But what if his alibi suddenly falls through? This adds another possibility for your readers to consider (or a red herring for them to follow.)
You want your ending to be a surprise to the reader, to have them say, "Oh, NOW I see..." If they've known for much of the book who the murderer is, you've removed the primary reason for them to finish the book...or buy your next one. On the flip side, you don't want the murderer's identity to be a complete shock (you can't get away with a suspect who is introduced to the reader in the last few pages.)