In Batman Year Two, Batman takes Joe Chill to the alley where he murdered his parents. He shows him his identity and points a gun at him and claims he will kill him. At that moment the reaper shoots and kills Chill. Was Batman actually going to kill Chill? Has Batman ever intentionally killed anyone before? Batman then goes on to battle the Reaper and he uses the gun on him, so that's one rule of his he has broken.
UPDATE: I believe whether Batman has a rule of killing or not varies heavily depending on the continuity. So for this question let's focus on the continuity of Year Two/Year One.