MC went from "The weakest hunter" (which surely would have built an ingrained "loser" mentality in him) to a super-powerful pseudo-psychopath (as he loses bits of his human emotions).
I'm really not surprised that he's highly suspicious of a girl who approaches him, not as an "older sister" nor a "younger sister".
If the author is so inclined to do so, she could be the "anchor" to keeping him "human"/humane.