In reality, even if Su Jie had not killed Di Xiu, because he forcibly burned his Holy Blood and transformed into a Sea of Blood, the Holy Blood he burned would have been completely depleted afterwards. His strength would have drastically declined, essentially reducing him to a crippled state with tremendous cost. It was not desperation, he wouldn't have done this.
In Su Jie's view, the Blood Sea Devouring Sky Demon Skill was essentially a Body Refining Technique that emphatically strengthened the physical body, allowing one to shake heaven and earth with the physical body alone.
As for why the Blood Sea Devouring Sky Demon Skill was classified as a Demon Technique, it was simply because cultivating with one's own fresh blood alone was far too slow and inefficient.
