What mental imagery does 如麻 invoke?
in literatures of yore, the term "殺人如*" has several variations.

in 陳拾遺集卷九


  殺人如麻﹒血流成澤


in 太白詩　白馬篇


  殺人如剪草


in 建炎以來繫年要錄　卷四


  殺人如刈麻﹒臭聞數百里


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.