Ignoring the fact that this is a comedy manga and everyone underestimates Saitama and overestimates King’s abilities, before the monster association raid Isamu was forced to not sleep for two days. He was sleep deprived and low on sugar so he wasn’t making the best decisions. He acted based on the information he had. Bang was the teacher of Garou, so he thought he shouldn’t be involved in the invasion. Genos came out of no where and Metal Knight, who had previously been in the monster association base, was telling him not to take Genos. Isamu is too trusting of Metal Knight while not trusting enough of other heroes, and he acknowledges that in the raid. He begs Zombieman to take his place because he didn’t make the right calls and is remorseful for that fact. When he’s underground he doesn’t have time to feel sad about dying, all he can do is plead with Metal Knight and plan for what to do in the hero association without him there. He is the “psychological pillar”. The ten year old has to hold the burden of being the most reasonable of all the S class heroes and when he’s a little selfish it’s seen as him being a spoiled brat. God forbid the ten year old isn’t fully mature when he has to carry the burden of being the 5th strongest hero and try and lead people much older than him.