The first name Bakugou gives himself is “King Explosion Murder” which Midnight rightfully remarks as being unacceptable. Bakugou takes a moment to reconsider, but the part of the name he changes is what’s interesting.
The part he chooses to alter is the noble title attached to the “Explosion Murder”. While some might just see it as an alteration of the exact same word, let me and my research on medieval history shed some light on this small detail. Within the European Feudal System, the nobility went in descending order of King, Duke, Count, Earl, Baron, Lord. Out of all of these, a Lord was the lowest rank a nobleman could hold, often serving as a medieval equivalent to a mayor, or if he was even lower in stature, he might only be Lord of a single manor house, ruling only over workers and peasants who plowed his land. Had Bakugou chosen to change his hero name to say, Emperor Explosion Murder, he would have been elevating himself to an even higher noble title, but instead he took himself from a king to the lowest position within the noble hierarchy that he could. Heck, even many barons were of extremely low power and significance, some even only holding the title in name with no actual legal responsibilities or lands to govern in service to the crown. As such, a mere Lord is a far cry from the all powerful king title he held before. This means that when Bakugou looked at the hero name Midnight disapproved of, the part of the name he thought she found inappropriate was calling himself a king, and he willingly lowered it to the most lowly and humble rank within the aristocracy. While, sure, he didn’t humble himself down to the level of a gentleman or a yeoman, but it still shows a level of humility and self-reflection that is easy to overlook.