If you rig an atomic bomb with a 10-second timer, initiate the countdown and then kill yourself before it blows, is that still mass murder, or just a suicide followed by complications after dying. A suicide bomber gets blown to bits before the shock wave from his/her bomb hits other people and kills them. I doubt "instant" suicide is possible, the key is to find a way that kills you faster than your nerve signals are able to respond.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram