I looked at the novel and it seems like they rushed through the first part (these 6 chapters covers 50 chapters of the LN).
Not that the LN is that great either but this adaptation is especially sloppy.
@kronous slight spoiler I guess but essentially his power has two "modes" an active and a passive mode. The active mode can only kill someone or something he knows about, but the passive mode kills anything that is attempting to harm him without limit.
I think its sorta implied the passive...
I doubt someone like Ousen would lose to a sneak attack. Even if rinboku actually caught ousen unaware he still should be strong enough to deal with it without help.