Non-violence can be absolute with no violent action allowed without exception or it can mean something less elegant. Perhaps they see it as building a more peaceful world which may require self defense or defense of innocent people. It actually makes good sense that the deadliest people in a society are also the most enlightened. Wouldn't it be great if all police officers meditated for an hour every day? Shao-lin were non-violent in other ways like their vegetarianism. To answer your question they justified violence for survival and something like "the greater good."