i don't know if anyone will read or care about this analysis, but what the fuck, ramblers gotta ramble. in the wake of the 2nd place piece in the art contest, Horikoshi hesitant to uphold it as a canon interpretation of bkdk's childhood, and all of Noguchi-san's art...
bkdk weren't good friends in their childhood.
and that's kind of the whole point. that's why Katsuki not taking Izuku’s hand in the creek is his biggest regret - had he realized up front that Izuku wasn’t condescending him by offering help, had he the humility to accept that Izuku was naturally the more empathetic hero between the two of them, then the two WOULD’VE BEEN good friends.
yes, even while Katsuki remained a little shit as a child, bkdk could've been very good childhood friends. I'm a firm believer that despite all the envy and miscommunication, that Katsuki has always cared about Izuku - even if for nothing more than his dependence on Izuku’s admiration - and that he's always lowkey admired Izuku’s drive, that he's always thrived off of the way Izuku challenges him
if Katsuki didn’t already care so much about Izuku, he wouldn't have taken Izuku’s presumed condescension or dishonesty about being quirkless so personally. for all we know, Katsuki might've wanted to keep Izuku as a friend, and saw the creek incident as a betrayal
but Katsuki’s rejection of Izuku that day set the tone from then onwards, and now Katsuki’s been scrambling to not only atone, but make up for lost time. and to that end, it makes sense that he'd panic so hard over losing Izuku - he'd be left to grieve over what could've been if he wasn't such an arrogant prick
the second Izuku gives him the greenlight that he too wants to be friends, not just rivals, Katsuki’s gonna go all in, I'm sure of it
that's a huge reason why bkdk is so fucking fascinating - Katsuki’s desperation to build what he and Izuku SHOULD'VE been all along