I really love the tone of this manga, its its funny and have a dark tone, but not dark enough to ruin the vibe. Sure they are suffering, but thats the point, it gave them a reason for them to stop becoming an assassin. Heck, even that chapter when Konoha kill that girl that kinda looks like her didnt really ruin the vibe for me.
I think if they killed preexisting character is when they cross the line for me.