I find that fear is quite useful in getting humans to walk the virtuous path.
It is true that the manga has no qualms showing evil like that, but I do believe that this is a continuation of the false paradise arc with the neutral souls from earlier. The main problem with the neutral souls from...
I believe the common thread among the pure souls and the tainted souls is selflessness vs selfishness. It is of no coincidence that the pure souls Raoul rescued in the chapter had concern for the other tainted souls, whereas the tainted souls had concern only for themselves. This also explains...
Going by the Bible, Lucifer and a third of the angels fell when they started a rebellion against God, not when Lucifer was thinking bad thoughts. Mayhap it is the same here in this story.
Also to add to Glomoro's post, said sister that the MC likes ran to the MC for protection, as the Feng family clearly cared less that said sister was alive. They even pretty much told the sister's mother as such. The MC has plenty of justification for treating the pregnant concubine the way she did.