Maxwell writes:

ammonium nitrate, dip****.

Ammonium nitrate won't do it. I asked about something readily accessible.
Something convenient and easy to use, in other words. Something that kills at
a distance without endangering the killer. Something that can be used on the
spur of the moment.

Ammonium nitrate, even when used as a bomb ingredient, does not meet these
criteria. There have been very few mass murders carried out by individuals
with ammonium nitrate.