I didn't remember the MC's attitude deficiency from the anime. But on the other hand, if he has grown up hearing every day he's a failure, one in a million mutant without the skills even a small child should possess, it makes sense he would grow up to be like that. If the world doesn't care about you, you don't care about the world. That's how it goes. He basically only has his little brother on his side, suffering from the same fate. The girl he was interested in ended up with another man with a brilliant future, not one with no future like he. Psychologically it makes sense he would want to hit the road immediately and leave the kingdom to look after itself. Even if the old classmates died, it would already be old news once he heard about it.