this was going to be a quick meta post about bakugou and self-confidence in general, but i ended up having too much to say just about his expression in this one panel from chapter one lol

this is his face the first time bakugou realises that he was wrong. that he underestimated midoriya, that he needed help, and that he’s not strong enough to fight of a villain completely on his own. this also forces him to see the gap between him and all might, who just earlier in this chapter he boasted about rising above. bakugou’s never ever had to question his own strength until this moment, something that can largely be blamed on the adults around him and how they treated him as he was raised. regardless, for the first time in his life, bakugou is made to feel weak. i think that’s the beginning of a central theme in his character arc.
when he chases midoriya, we see for the first time the difference between what bakugou says and what he feels. with the pro heroes praising bakugou, from what we know of his character already it would be expected for those words to further inflate his ego, but instead of boasting or accepting the praise he has this expression (see above), frustrated and uncomfortable. along with that, we can guess this is the first time in his life bakugou has been in such a helpless and scary situation, which further acts to give him a reality check of what his strength actually is when measured against other people.

when bakugou chases midoriya his first words are “i never asked for you to save me,” while shaking and talking quickly (how someone might say something to convince themselves more than someone else). assuming the above, that he’s saying it to convince himself, this is the first example of bakugou equating asking for help with weakness. it’s his first real experience of feeling weak, and denial is a reasonable reaction – but bakugou is so loud with his emotions and he’s very much an action-oriented person, so the way his denial is expressed is very explosive. he’s not only pretending that it wasn’t the case that he was asking for or needed midoriya’s help, but he goes as out of his way as he can to physically remind himself that ‘it’s the opposite’ – that by helping him or thinking he was asking for help, midoriya is looking down on him. he’s telling himself that midoriya didn’t help him because bakugou wasn’t asking for help; that midoriya wasn’t 'stronger’ than him because bakugou never showed weakness in the first place.
this is also the first time bakugou implies that midoriya is looking down on him. “how dare a quirkless failure like you pity me? trying to win me over? don’t you dare mock me!” – as bakugou understands it, midoriya tried to save bakugou because he thought bakugou wasn’t able to do something that midoriya himself was. i think it’s fair to assume that along with being deeply shaken by this experience, as well as midoriya’s involvement in it, bakugou is also forced to face the fact of his and midoriya’s history together; that his childhood friend who he’s aggressively bullied for so long was the one to try and help him, the one who think he needed it.
something important to note is what is implied by bakugou’s visceral knee jerk reaction to the events of the first chapter. ironically, we can assume by his violent denial of having needed or received that he is, on some level, acknowledging (even unwillingly or subconscious) that midoriya helped him. taking everything into account, already we can see the complicated cloud of emotions that surround their relationship with each other. from bakugou towards midoriya, there was a lot of disdain; he looked down on him, disregarded him, ignored him, not only didn’t see him on the same playing ground but didn’t recognise him as even playing the game at all. there’s also a feeling that he knows midoriya very well, them having been childhood friends. but now we have a hinted inferiority complex, there will definitely be confusion for why midoriya acted, resentment that he thought he could do something bakugou couldn’t, frustration that bakugou couldn’t do it himself, and we can even speculate that there’s guilt or regret regarding how bakugou has been treating him this whole time.
and alongside all of this is his knowledge of his own situation outside of midoriya – of his helplessness, apparent weakness, and how he measures up against all might and a villain. his blind confidence from the beginning has been shaken.

at the very least, bakugou has been forced to see that he was wrong about something: his own strength, the strength of what he wants to fight, the strength of who he wants to overcome, and even the strength of the person he thought was so below him they didn’t warrant acknowledging. for someone who’s never been made to feel that way in his life, it makes sense that it freaks him out – it’s the biggest reality check of his life so far. it makes sense that his reaction is extreme (and as we’ll come to see, it’s not something he lets go of or gets over quickly but rather it weighs on bakugou’s mind for months to come).
from all of this, we already know a lot about his character. the environment he grew up on in inflated his pride to bursting. he deals with things he doesn’t like by saying what he wants rather than what is – that is, he fakes it until he makes it. and we also see the beginning the complex melting pot that is his relationship with midoriya.
at this point, i also think it would be reasonable to start wondering: what is bakugou’s motive in becoming a hero? with his incomplete understanding of those who need help and the helper, as well as an attitude that nobody would call 'heroic’, i think it’s fair to assume that the author intentionally wrote it so at this point in the story, the very beginning, we would start thinking about what it actually means to be a hero and what that kind of person would look like. and this is a theme that is explored in depth through bakugou’s subplot throughout the entire manga.

another translation for the panel above is “so deku, the quirkless wonder, thinks he play hero, huh?” the very first panel in the manga starts with showing the differences between midoriya and bakugou. bakugou is our example of what it can mean to be considered a ‘hero’ in this society – strong, born in a position to be strong. it defines the role of quirks and the innate inequality they create, and defines bakugou, who’s just as intrinsic to our understanding of this as midoriya is. as midoriya ‘suffered’ in this society, bakugou benefited from it. i think the author wants us to immediately question from the get go is whether strength is enough to be a ‘hero’ – and if not, then what is? and that’s the question we try to find the answer to through the entire manga. what does it mean to be a hero? through bakugou’s character arc, established here in the first chapter, we find an answer that, when combined with midoriya’s, is nuanced and complex. they’re two sides of the same coin.
in conclusion; i wanted to zoom in on that one panel with bakugou’s incredibly troubled expression because it’s one we rarely see. he’s not aggressive – he isn’t baring his teeth or making any noise or even looking at anyone. it’s entirely reflecting internal conflict rather than external, and it’s a perfect launchpad to start analyzing and getting to understand his character. this panel makes us wonder, ‘who is bakugou katsuki?’ while simultaneously understand the answer to that is a lot more complicated than this first chapter makes us, on the surface, want to think.