Oh yeah, you could totally just leave it in a giant pool and ignore it. It'll react, evaporate and eventually break down into cyanide again, rain down, subtly poison the area, react again, evaporate again, etc.
And that's great for the owner of the big pool of cyanide, and very bad for everyone else. Stuff that evaporates doesn't disappear, the cyanide doesn't magically change into cookiedough. You're just spreading it around more.