The killer is almost always the person who is left out in the most of the investigation; the person you have the least amount of clues for and seems to have nothing to do with it; and the person who, when you are meeting the suspects, is met last.
It is possible to assemble and disassemble guns on the table. There are three parts. First, hold the Mouse over the top of the gun and move it left. The next two parts are the magazine. Hold the Left Mouse Button and drag the Mouse down. This releases the magazine.