Except by "killer" we mean "something that might get you mildly damp"; the only atoms fired by the Atom-Matic are hydrogen and oxygen, in a special 1:2 ratio. Yep, it's a water pistol. The Atom-Matic is also surprisingly hefty, on account of its metal handle, brass firing assembly, and brass tip, so in a pinch, you might be able to club your enemy into unconsciousness.
This is one of the rarer space guns, which makes sense; it'd be hard to imagine the abstract design proving popular with kids, especially in the 1940s when the toy market had yet to completely shift over to science-fiction based products (despite the popularity of Buck Rogers and Flash Gordon). That this gun even made it off the drawing board is sort of a miracle, and is testimony to the old-school toy manufacturer's willingness to recognize quality and creativity.