We re-investigate the global runaway instability of massive (self-gravitating) tori around black holes. For the first time, the metric and equipotentials have been computed fully relativistically from the Einstein field equations. We find that the instability exists in the few models that we have constructed. The runaway instability may provide the shortest evolutionary time-scale relevant for the current model of gamma-ray bursts based on the merger of neutron star binaries. Our rough estimate, based on the linear analysis of the stability of stationary tori, gives the time-scale for the lifetime of such tori, Deltat_T<<1s.