When a child, Midoriya didnt have any quirk. And at some point he considered Bakugo his friend. Why then Katsuki Bakugo felt threatened by Midoriya? This makes sense when Midoriya already had the One for All quirk and could be an obstacle for Bakugo to become the greatest hero (which is Bakugo's goal), but I can't understand why Bakugo was so mean to Midoriya when he was quirkless. Is there any explanation about this?
Katsuki Bakugo throughout all of My Hero Academia has a superiority complex, this is made evident by his lavish demands of not only Midoriya but also his friend group at the start. It's quite simple to tell he has this characteristic, as he is portrayed as being egotistical, arrogant and narcissistic. This kind of behavior has two spectrum's too it in character development winning or losing at the start he's higher in the pecking order thus he strikes down anyone as to show his superiority and fuel his own narcissistic traits. He specifically picks on Midoriya as he wants the admiration of his peers as being the strongest, if he picked on someone else it would be almost like saying they were stronger and he wouldn't potentially gain there admiration. Another take would be that Midoriya being born quirkless is actually kind of rare as there is always the potential for him to suddenly activate a quirk at any time, this unknown factor is a reason for katsuki to be unsure of if he's at the top of the pyramid, thus he bully's him to make sure he also believes he will never be strong.
This question as a whole is interesting because I believe there is a strong psychological meaning behind all of this that adds a whole dynamic to there relationship throghout the story so far.