It's been said many times how Crosscode's characters have a lot of unprocessed trauma and existential dread and I think Sergey definitely has it pretty bad. Two of his best friends are dead, one of which literally died right in front of his eyes and the other he never really got to grieve properly because Satoshi's Evotar is still alive and well.
Not to mention the fact that he had to deal with years of Gautham, Satoshi, and Shizuka being missing, disappearing after making their deal with Sidwell. And of course when he finally finds them, he learns Satoshi's been dead for years, and, days later has to watch Gautham die.
I also think Shizuka's attempt to break Lea and Lea's subsequent outrage over Sergey's lies had to have seriously hurt him too.
It's why I included his outburst(s) in Crossed Lines and am definitely going to be writing more about him processing (or failing to healthily process hehe) his trauma.