The Mutant Massacre was a pivotal moment in the X-Men history, one of our watershed moments. It is important to note that the story was written years before Gambit was ever introduced to the X-Men Books, somewhere around 1995-1997, it was heavily hinted Gambit had this dark history, so dark, Rogue could not even deal with it in her own mind until she was forced too. Readers finally learned in Uncanny X-Men 350 that it was Gambit who lead the Marauders into the Morlocks tunnels where the massacre took place.
How much of this was Gambit's fault? definitely a very complex debate. We all could argue reasons for and against Gambit's part in the massacre from him getting desperate and turning to Sinister for help, later feeling obliged to return the favour or Gambit was not naive enough to think the Marauders were going down just to have small talks with the Morlocks.
Sinister and his Marauders gets full responsibility but is it fair to say Gambit is just as responsible or just partially responsible?