As Kain started to face the lost souls, he began to realize why they were selected as his first trial.
Kain had thankfully not had to face many spirit-type spiritual creatures prior to Bea's evolution.
Prior to evolution Bea destroyed her enemies by targeting their brain—but what if they had no brain? She could still exert an effect by targeting whatever brain-like entity was responsible for controlling the body of the spiritual creature. However, she wasn't nearly as effective against them.
However, this became less of an issue after she evolved and instead focused more on manipulating and consuming mental energy made up of memories, emotions, and their sense of self. Now she kills by destroying mindshards—that is destroying the consciousness directly—rather than targeting a physical brain.