Working with chemical warfare is a huge risk.......Batman doesn't know the long term affects they might have, or health risks. Essentially, think of it as a drug that never got approved by the FDA. Bats could do more harm than good. Not to mention that if he slightly slips too much of a dosage (even just a little) it could send some guy to jump off a roof out of pure fear. Or, he could infect himself or even Alfred with long term damage of some kind.
Basically, too risky.....he could cause massive insanity within himself.
Now.....that's the technical answer. The real answer is, the Goddamn Batman doesn't need it. Fear Gas is, clinical tests have shown, actuall afraid of the Goddamn Batman. Wow. Wowwy. Wowwy Wow. Wow.