That is true, but Gallagher does not feel he is stable enough to handle killing at this time. All Jedi kill in pursuit of completing their missions and to enact justice, but in this situation Gallagher cannot be sure how killing would help. Killing Sienar would destroy everything and initiate the famine, but killing his Jedi companions, assuming they come for Sienar with lightsabers drawn, will only take him towards the Dark Side.
Unfortunately, this may change as Gallagher finds out more about what Sienar is planning and how his existence is connected to the potential famine.
But he certainly doesn't want to kill his Jedi friends, or Sienar.