People use the idiom 杀人如麻 all the time but hemp is no longer a familiar scene to most people, and I doubt anyone ever saw a hemp in his whole life. Exactly what mental imagery is 麻 supposed to invoke?
in literatures of yore, the term "殺人如*" has several variations.
in 太白詩 白馬篇
in 建炎以來繫年要錄 卷四
i would make a mental image of "殺人如麻" as "killing people like mowing hemps". such metaphor would be:
sickle - the sword, knife
mowing - killing (people) without considerations, massively
when mowing hemps, or grasses, no one spend time to examine particular grass, appreciate it's beauty; or consider the rationale of mowing.
hemps - life is worthless, without value
well, in mowing, one would consider cutting / clearing as many hemps / grasses as possible, with minimum efforts.