Haha, you are right for the previous episode, but in this one it makes kind of sense. If you are that powerful, they don't want to heed your words and you can get a new group of heroes after killing them, killing them would be a really good option and the fastest one at that (though you would get weak af heroes in exchange.)
Yeah, if anything I appreciate her "ends justify the means" attitude. Not many characters are willing to take the more brutal path to doing what's best for everyone, instead choosing a less effective method so that they can feel better about themselves.
The world doesn't need hashtag do goodism to fight evil, it needs someone willing 2 get their hands dirty. She only thinks she knows what's going on. Killing the heroes would only pass the problem to the next guys, it wouldn't solve it. Myne would kill melty then would do what she did before only better. Her "help" would be no help at all.
You seem to be missing that literally none of this is Naofumi's fault and instead the fact that they all ganged up on him when some planted evidence and a red head comes up crying about falsely accused rape. Now he's going to have to die just because the others despise him despite how useful he was and how, time and again, he proves to be the best one of the lot? Nah, she ought to 'convince' the others first at least.
Kain said in soul reaver 2, "there's no shame in it, Raziel. Revenge is motivation enough, atleast it's honest." Does it matter if it's revenge? In real world medieval times no one would care. He would have kept killing.