Ikk people are just ignoring the story and taking the characters out of context and getting mad like Abel stoping Karas is completely in character for him it is literally his personality to look out for others more than himself He values life.(His reasons justified) even tho it can be naive and unreasonable sometimes.While Karas personality is that he doesn't care much for anyone including himself he doesn't really value life.(His reasons also justified) even tho can be crude and cruel sometimes. Both personalities are are extreme polar opposites and are going to clash constantly. When they're together they'll eventually balance each other out; but we have to get there first through trial and error. (┛◉Д◉)┛彡┻━┻
Abel isn't even stopping him from killing the fake though. Abel is stopping him from killing the hostage which is the person he is comparing to an innocent bug.
Karas is willing to cut through the hostage because it's "faster" and Abel sees that when the first hostage gets hurt and barely makes it out from being killed.
Ik that's why I said sometimes I do not feel like going into detail again about why here's it simple
Able knows it would be unreasonable to spare the imposta due to what he's done but knows it unreasonable for Karas to hurt others to get to him and traumatize others while doing so. He would spare both to prevent unnecessary bloodshed and death because there is better ways to handle it rn . That's part of their personalities. Karas sees the hostage as an obstacle and is ready to cut them both down he doesn't care about the life of people he doesn't care about. That why able and co is there to bring him to a more reasonable solution.
Abel doesn't like any killing whether it's a friend or foe. He values all creatures (even bugs) so saving a life comes naturally to him even if it means putting himself in danger That's how noble he is.
He should stop dragging sir Melmont through the mud unintentionally tho