NOT EDITED YET

Everything made sense now

If the three pillars already knows where the remnant power of the Demon Lord resides then shouldn't it be easy to completely eliminate it?

They just have to send a person of their choosing and eliminate the threat which in this case was Lawrence before he can even engulf the world in chaos

Of course, after Lawrence's mortal body was killed, he would then transform into this demonic pillar

Only a person with strong willpower can wield and use the sword of salvation. If the chosen one is Cain then it makes sense that he has to grow first to the point that his willpower is strong enough to wield the sword

However, Cain is not the chosen one. He is just a tool that was used to hone the chosen one

Cain's willpower mostly comes from his talent and also the terrible fights that he had in his life