The difficulty with toxic gases is in the delivery method. For example Chlorine gas at hundreds of PPM still takes several minutes of inhalation to be fatal, and it is a strong irritant so everyone will know it's there and be trying their hardest to get away from it. So you either need to trap someone in a very enclosed space or release ridiculous amounts of it very quickly. Viable for trench warfare, but not much else.

Realistically you need something odorless (=exotic enough the human nose hasn't evolved to detect it), fast acting (=highly reactive/unstable), and dangerous at low concentrations (=likely to kill you while making it).