In general, since the topic of Akira's childhood has come up, I think it's worth mentioning his family. First, Akira's mother Saki. As far as we know, she didn't like him very much and I think that she most likely tried to spend as little time with him as possible, which naturally pushed him even more towards Sekai for the feeling of care and love (which she took advantage of). As for the father, he is the hero of the last game of Celebus and seems to be even more crazy than Sensei (although I didn’t play, so I don’t know for sure) and it’s quite possible that he didn’t really care about Akira either.
I won’t say anything about Nozomu - many analyzes and assumptions have already been made on him.
Summing up, I think that not only Sekai (although mostly she) but also his family is to blame for the worldview and state of Sensei.
P.S Now that I think about it, the only truly good people in his past were Nakayama's family.