I'm not too sure what the point of the backstory is, since it sure doesn't make them sympathetic. I wouldn't reject a plot where traumatized kids with superpowers have decided to take revenge through violence and we're expected to empathize with them, but after showing that they actually find killing people fun in chapter 51, I don't think that was the purpose.
Maybe it's just to show that even though they like killing, this didn't come out of nowhere and they had reasons to turn out like this? That addition would make sense, I guess.
I just hope I'm not wrong. If this all turn out into a "they were victims first, let's support them and just forget about how happy they were about killing before", I'd find this pretty annoying.