The central mystery of Heroes in Crisis #8 has finally been solved, as the architect of the mass-murder at Sanctuary is revealed.
Warning: this article contains spoilers for Heroes in Crisis #8!
Despite early evidence pointing to either Booster Gold or Harley Quinn as the culprit, and despite the fact that he himself is included among the dead at Sanctuary, Wally West is revealed to be the true killer in issue #8. Wally methodically lays out the series of events that led to his psychological breakdown, the deaths at Sanctuary and the resulting cover-up.
Source: IGN.com DC's Heroes in Crisis: The True Killer Finally Revealed