Huh. Beryllium is a tricky one for insta-death. Berylliosis is a longer term disease and not everyone is susceptible. You could certainly make an arrow or falling object out of it though.
Probably more in the spirit of things, you could use the fact that it is the metal with the very best heat dissipation by weight. Maybe a fire behind a door with a Be knob, so the victim doesn't feel the heat when he turns the handle. Or some such.
Boron? Equally boring. Except as borane gases, which can be both toxic and flammable! Or in bleaches, which you could liberate chlorine from. But neither is really a boron-only kill.