As a hardcore Ichi the Killer fan, I deem "1 (1993)" to be an extremely important read before "Koroshiya 1 (1998)." If you would like to see it via film, please check out the live-action film: 1-Ichi (2003). Be sure and watch it before you watch the live-action film: Ichi the Killer (2001).

1 (1993) takes place during Hajime's School days, before all of the depraved, gore-filled blood shed depicted in Koroshiya 1 (1998). Hajime's (Ichi) day-to-day experiences with social ostracization, discrimination, and hardcore-bullying, in addition to being manipulated via brainwashing in the sequel are the byproducts which molded Hajime Shiroishi into becoming the killer that he turned out to be. In this manga, much alike the sequel, he's an introverted individual that likes to keep to himself. He likes to attend martial arts practice to master his deadly bone crushing kicks, and does what he needs to do to survive both academically, and quite literally. Both the manga and the live action (1-Ichi 2003) serve as a way of humanizing Hajime, or Ichi, without much murderous glamorization. In turn, this shows that he was not born as the depraved murderous lunatic that people made him out to be, rather, to show that people can only take so much humiliation, abuse, and torment until they snap, and ultimately lash out. This manga serves as a prequel before Hajime, or Ichi became a brainwashed hitman, and is very, very important to read before Koroshiya 1 (1998). This and the sequel are two of my favorite works of all time, and I can read them over, and over again. Highly Recommend; 10/10.