@Kuyu
They may believe him or at the very least be ready for it.
The problem is can he completely trust them, he met them after 12 years and don't know how it has changed them.
Plus they aren't the strongest guild around, this disaster can easily be blamed on them by Ares if they ever opened their mouth that they knew it was going to happen, and the general public will happily follow along to blame them. And even if they don't get blamed for it, they would be expected to know when all such disasters take place and are there to fight it as some kind of heroes when even the largest guild isn't held up to the same standards. Basically, the public mules being fed people's praises and also expect others who don't like their prestige to sabotage them.
As for MC being self-absorbed/ selfish, you have said it yourself he saved plenty of people in the tower out of boredom. Would like to point out in flashbacks he was a ready and willing participant in facing dangers.
But don't you expect him to cope with the dangerous environment in some ways or he would be bonafide crazy or even commit suicide, plus after helping plenty of people and seeing he couldn't leave with them wouldn't an intelligent MC try to find some other way or do you just expect him to people all the time while he stays in the tower.
As for saying some Koreans are self-absorbed/selfish, what did you expect really? There are these kinds of people everywhere and there is no way to generalize them by race, sex, ethnicity or nationality.
I personally have met some really nice Koreans and some whom I won't feel any regret calling a-holes or piece of $h*t.
In conclusion, the MC never said he was a hero or saint as well as he is just learning everything about the outside world which has dramatically changed after 12 years of improvement in tech and magic, and a new political climate. Him doing thinks cautiously while making sure his enemies under-estimate them is far better than some story where the MC is ready to put his life on the line for someone he just met a second ago due to some hero complex while not thinking ahead.