My mental gymnastics here are that, while Finn was programmed from childhood to be a soldier, he was not isolated. He would have ample opportunity to read, learn about the wider galaxy, use the internet (holonet?), and talk with others. Plus, the specialization he had (sanitation worker) means he didn’t get a uniform…

There’s a key difference. The people on Jakku were defenseless. The Storm Troopers he blasts are armed, trying to stop him, and part of an army that kills defenseless people.

Nah. He was supposed to have been mentally conditioned from childhood. I doubt he was conditioned to have a friend or even have empathy because if you want people to follow orders blindly, the last thing you want is for them to develop personal attachments.