Eh, I'm a bit willing to give a bit of a pass on the 'it would be murder!' stuff, as 1. Abel rendered the guy a non-threat so it was no longer self-defense as they could have walked and gotten other people, and 2. It seems implied to me that the guy was under some kind of corrupting mind control stuff. Between not only his extreme behavior now, the signs of being unhinged when we last saw him preeching anti-magic stuff, and also how this seems to be hinting at how insanely quickly the human world declined (within about 3-4 generations from Abel) is in part due to demons working from the shadows to suppress human magical ability which all of that would require some mind fuckery an an extent, it's just a lot less clear cut.
That said, just because this instance of hand-wavey bullshit can at least be argued, does not mean that this suddenly stopped being a relatively trashy harem manga.