Writing a Synopsis

Synopsis-writing is tricky. It differs from jacket summary writing in that you aren't trying to entice the agent/publisher by making them want to know who did it, but rather how you tell a story. So yes, you do need to sum up the story including the ending. Trying to keep the feel of the story and your writing present in the synopsis is the best thing you can do. Also write in the present tense, keep the active voice.

      I can’t tell you how important it is to give your synopsis a sense of your voice. It's not just a regurgitation of the plot. You must intrigue the reader into the story, show the key suspects without going into details of the investigation, and provide a general feeling of how the book will end. In my opinion, it's not necessary to walk the reader through every chapter, blow by blow. But it is important not to surprise the editor/agent with a weird twist or off the wall suspect that comes out of the blue.

       All the natural questions should be answered so the editor/agent doesn’t have them in their minds. The main objective of a synopsis is to pique the interest of the editor/agent and get them to ask for a full manuscript. That's why voice is so important. Seduce them into wanting more and let your writing sample do the real talking.
     The killer and his/her motives should be part of the synopsis, in the last paragraph or two. The best advice I ever read on this subject was that you should never say anything like, "To find out more, you'll just have to read the whole book!"

A synopsis, short or long, should tell the whole story, including the killer. With the synopsis you give enough details of the climax so the agent or editor reading it knows exactly what happens. They want to know from this that you can finish the novel in a satisfying manner and that the ending works. Agents/editors hate it when you tease them. So spell it out. Generally, with a synopsis, they want to know whodunit. They want to know that you can deliver on the ending.