There are many facets of Batman that make him formidable. While his training and gadgets are important, his skills as a tactician are what really make him stand out. His ability to outthink his opponent is what elevates him to superhero status. However, Batman is as paranoid as he is brilliant, and that means he always has a backup plan. He is always prepared for the worst, even within the Justice League of America. This paranoia is on full display in the comic JLA: Tower of Babel.

In the Tower of Babel, it is discovered that Batman has been keeping a hidden file on all the strengths and weaknesses of his allies in the JLA. The story follows what happens when Ra's al Ghul steals the file in order to defeat the JLA.

Batman's rationale for having the kill file is that if any of his friends were to become evil, he would need to have a way of defeating them. While some in the superhero community sees this as a betrayal, Batman sees this as a necessary evil. It is a way to prepare in case the worst case scenario happens. This gives us a great insight into the psyche of Batman. No matter how close he is to someone he will never completely let his guard down.

Here's how he would eliminate the key members of the JLA if he ever needed to:

Superman- Everyone knows that Superman's one weakness is Kryptonite, so Batman found a successful way to weaponize it, he created Red Kryptonite. Red Kryptonite is an artificial creation of Batman's made by exposing a Green Kryptonite sample to radiation. It was developed in the event that he needs to incapacitate Superman without killing him. Superman's skin would become transparent after being exposed to Red Kryptonite, causing him to feel intense pain. This would also overload his natural solar absorption to the point where his super senses become overwhelmed.

Wonder Woman- Wonder Woman is known for her intense determination. Batman's plan if she turned evil would be to inject a nanite into her ear. This would cause Wonder Woman to be trapped in a virtual reality battle against an opponent whom she cannot defeat and is her equal in every way. Her refusal to surrender under any circumstance would eventually cause her to tire herself out and die of exhaustion.

Green Lantern- The ring worn by the Green Lantern is an incredibly powerful weapon, however the Green Lantern needs his vision to guide it. Batman has figured out that if need be, he could render the Green Lantern blind by his own power ring from a post-hypnotic suggestion introduced during the REM phase while he is asleep. If he believed himself blind, the ring would then make him become blind, due to his own will power.

Flash- The Flash's speed and healing ability make him a difficult opponent for anyone. The key to beating him is a specially designed "vibra-bullet" that would strike the Flash in the back of the neck, causing him to experience seizures at light speed.

Martian Manhunter- Martian Manhunter's weakness is fire. However, fire is difficult to use in a combat situation. Batman discovered that if he needed to defeat Martian Manhunter he would use a nanite virus spiked with magnesium. This would convert the outer layer of Martian Manhunter's skin into magnesium and cause him to burst into flame when he is exposed to air.

Aquaman- Aquaman obviously needs water. If he ever became aquaphobic he would die in hours. Batman decided that if Aquaman had to be eliminated they would use an altered form of the Scarecrow's fear toxin to make Aquaman afraid of water.

Plastic Man- To defeat Plastic Man, he would need to be frozen with liquid nitrogen and then shattered.

Last but not least, Batman himself. Batman is so concerned with keeping the peace that he created a file to help eliminate himself if he turned evil.

Batman- While Batman is a master strategist and combatant, he is still only human. The way to defeat him is his human weakness. In his file he says the best way to defeat Batman is to distract him, either by taking hostages or by exploiting the blind spot that is his parents. In JLA: Tower of Babel, The League of Assassins steals the bodies of his parents.

Only a true hero like Batman would do whatever is necessary to do the right thing. Batman knows the hard truth, that he will have to do everything he can to defeat his closest friends if they ever become evil, and this file would help him with that harrowing task.