Well, I don't mean to harp on this and I keep this into a generalization, b/c it's a little left field. But just because taking out a bad person who'll just have someone else just as bad, doesn't mean that the first bad person shouldn't be taken out of power. To me, that's just a defeatist way of thinking. By that logic, why do anything to try and make the world a better place; you may as well sit there like a bump on a log. I'd take Grey's assertions with a grain of salt, since he is benefiting from things being the way they are.
Though, yeah it is pretty interesting that Grey tries to be careful with how he provokes Marcone because of how mortals can be dangerous. Does Grey make any mention of Monoc or BFG as a reason to play nice?
Hm, as for 'Changes' I would posit that Harry's friends do bear some measure of responsibility, at least in the sense of enabling his 'burn the world for the sake of one person' attitude. They could have at least tried to talk him down or curb it. It wouldn't have worked, but it would have been nice to see them try. I'd say that while Molly had choices as well, the fact that Harry is her teacher and is in a position of power over her means that she isn't in a strong place in this scenario.
Yeah, 4) is a bit thin. Though Bianca didn't have to kill her secretary and could have exercised enough self control to not kill someone. Though by that same token she'd just end up losing control and killing someone else. It might be someone she won't get as upset about it. Maybe even to the point of not holding a grudge with Harry, but either way there's still a dead person.
I hope that at some point Harry can have enough character to development to say, 'no this is not my fault' with something.