[bell-cot]

I'll see your CH3NCO, and raise (CH3)2Hg -

https://en.wikipedia.org/wiki/Dimethylmercury#Safety

- considerably more deadlier, and the mercury an element - so no clever chemical reactions can break it down into innocuous CO2, H2O, and N2.

[amluto]

True. But methyl isocyanate has the regrettable property that it was quite useful, to the point that there was a crappy, poorly maintained factory with tanks of it on site, with extraordinarily regrettable results.

https://en.wikipedia.org/wiki/Bhopal_disaster