Shuu's been disappointing? I think one of my favourite things about this manga is that Shuu actually feels like a naive vunerable kid who just happens to have knack for killing people and isn't just some generic self insert power fantasy edgelord.
I agree with Emmypacer I actually really like how helpless Shuu was. It's been established that he's unkillable at range and his PTSD makes him too scared to do anything up close. Why should he suddenly be able to take out 3 his with guns while unarmed?
Man, every time you look at Shuu, you can't help but think he acts like his father as his own way of coping. God, it's really sickening to see him so helpless. It's just that powerful.