you say this based on what? certainly not based on what you saw in the movie. in the first half of the movie, you see that batman flees the League's headquarters with Ducard during a fire... yet doesn't save the chinese murderer in the cage does he? in your mind that means he's that guy's executioner as well? (logic be damned... and the fact that the only definitive action he DOES perform in that scene is to refuse to be the guy's executioner)
no. you think Ras' style is to rat out batman like some 2 bit hood? you think Ras thinks the police have the AUTHORITY to settle the matter between him and batman? if anything Ras would do everything to make sure the police never piece together who batman is because they are corrupt and weak and the only way to deliver JUSTICE to batman is to mete it out himself.
what distinguishes Ras and Batman is NOT that Batman does not kill. it's that Batman refuses to make himself the arbiter of justice. Batman knows that justice is about more than what he personally wants. on the other hand Ras believe only HE can deliver justice but in fact one man's desire to punish others is merely vengence
Batman knows that he's not qualified to be the judge and executioner. there are ideas larger than him... society, God, law and justice means that the fate of evil doers is left to higher powers. there are laws for every 2 bit hood stealing a purse. but what is the sentence for trying to obliterate an entire city. who is qualified to judge the attempted murder of 10 million souls? if he sends Ras to jail knowing that Ras will easily break out in days is that justice?
The core of Batman is that "the wicked come to evil ends." Batman is a man who at his core believes 100% that evil will be punished and the innocent saved. if Ras is meant to die on the train he will... if he is meant to live he will. that's why batman left him on the train