From what I get from it, Zoro is referring to life. Note it's "on the path of carnage" and not "on the path to carnage." In buddhism, life is often painted in a dark light. To reach nirvana, you are beaten and reborn until you reach enlightenment. To be honest, I think that quote goes hand in hand with his battle with Ohm. From it, he attempted to cast out his attachment for Chopper. Suffering is rooted in transient concepts and passions, like life, love, anger, desires.
Asura is Zoro succumbing to his passion; therefore suffering, as Asura was a deity defined by his passion, and why he is tied to war, which is suffering born from passion. It could explain why he has yet to use it post timeskip, and part of it has to do with his training, which was compared to enlightenment stage from the Bodhisattva Path theory
@MasterD helped craft.
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Well, Rengoku means "purgatory", which is the state between life and death of where sinners go after they pass away. At least in christianity.