If your hero/detective is supposed to be legendary [like Sherlock or Hercule Poirot] then the ending has to be extremely gratifying. Each of the clues needs to fit into the punchline. A lot of writers do this by making clues to pieces of the mystery, for example, a messed up floor is at first something that seems like a clue to the main mystery, but instead is a clue to how somebody got to a window, which they then had access to frame another person. Something like that. You can also keep clues to other things, such as character development. An interesting thing you've likely seen a lot is a piece of knowledge that someone would only know if they were a doctor, or a soldier, or what-have-you. That's a really cool way to allow your hero to make a guess as to a murderer, without making it obvious for your readers. It also lets your readers from that world gratified, because "who could know this! I'm special and good at solving mysteries". And that's a feeling you want. It's like a hard crossword. If you can solve a really difficult crossword or you're watching jeopardy and get every question right, that feels way more gratifying than completing anything easy.

Other commentors are saying you should write the story after a full plan, which is fine, but that's not the only way to do things. You can create your points of interest in the story—you have a basic idea: A person [or more] dies, a killer [or killers] kills, and you have a horror element 2/3 way through, there's the setting for the first 15 minutes, person 7 gets a past event explained at this point, etc. Then you can mess around with the story until everyone is the murderer. Like in the other commentor's teded video, Agatha christie often switched who the killer was midway through. This allowed her to keep it fresh and also let her make sure that the novel is completely indecipherable until the very end. You're likely to give it away as you go just by adding the clues you're already adding. I don't think you're just going to create a random story and then magically solve it at the end. It's a lot harder to give it away if you don't even know who the killer is though.

If you go this route you might spend a lot of time rewriting and adding; meaning your book is going to be good.

If you like you can also add a neat twist, such as maybe your mystery knows who the killer is but not who the victim[s] are. People might like that, but if you aren't up to it you don't need it.

Make your characters captivating