To be fair realizing that he is not meant to be a leader but a soldier is probably the smartest decision he made. Just because someone is a great fighter doesn't mean they're a great leader. Him passing it off to Yumina who is able and willing to get her hands dirty and make the tough decisions is actually pretty smart... he probably should have broadcasted that decision though so everyone will know and not cause confusion.
It's 100% on Katsuya for not issuing any orders. The chain of command collapsed and people started doing their own thing which lead to a lot of people being in splat range instead of maintaining their relatively safer original plan.
