Batman isn't too far off from becoming a psychopath. He's stated over the years, as have others, how narrow the beam over darkness he walks is, and how easy it is to fall from it, plunging himself into a place he can never return from. Taking a human being's life, no matter how evil, where you are in no position to do so, is always going to be perceived as wrong, and rightfully so.
Batman bends the law, sometimes he breaks it, all for the greater good... and this is for the sake of upholding the law. He has to draw a line somewhere, because if ge doesn't, what example is he setting to others out there?
Even after Commissioner Gordon has been tortured, seen pics of his daughter naked and crippled, does he tell Batman to kill the Joker to prevent this from happening again to anyone else? Does he ask Batman to beat the clown to a pulp? No. He instructs him to bring him in by the book, to show him that the lawful side of his character will remain intact, despite everything.
Batman, like cops, works alongside the rules of law and order. Police officers don't go in with a plan to kill a criminals, it's to arrest them. The world would be so much simpler, but where would that leave our society?
Batman, being so close to evil, because let's face it, he's not exactly normal, is he? Should absolutely be the first person to recognise a place to draw a line, so he doesn't run the risk of being like those he fights, who have no regards for law and order, life and death.