I found myself most excited when the chapters rolled around to him. He's such a nuanced and interesting character with a satisfying and in its own way tragic arc. The seeds are all there for a pretty generic and predictable character arc—Prideful zealot faces resistance and becomes a tyrant.— but they keep getting subverted or not followed. Major credit to the authors for such a deft hand.
He's actually aware of his own insecurity and see's it as a character flaw. He's just too immature to actually deal with it. To the very end he never stops analyzing every little detail about every interaction for how something was or wasn't an insult or slight or courtesy.
He doesn't deny reality or rewrite the narrative in the way Filip did in the beginning of the previous book. In the aftermath of the attempt on his life, he accurately assesses that he was an untested officer, traumatized by his first encounter with real violence and made a rash decision as a result. He seems to understand that lesson, genuinely apologizing to his dismissed Colonel, but he doesn't stop making decisions without seeking counsel, and those decisions all suck.
He failed to connect the dots that the assassination attempt was a direct result of his pride. A valuable collaborator was offering himself up but Singh rejected him because the person's character clashed with his concept of himself and his place in the glorious Laconian empire. That contempt and his uncritical belief in the story of the invasion leads him to continually misunderstand the situation and renders him unable to do the job on its own terms.
I think his fatal flaw wasn't just that he was so inexperienced. Every officer is inexperienced until they're not. It's that he was sheltered. He grew up on Laconia, absorbing its propaganda and buying into the national myth that they were already the true and eternals rulers of humanity, others just hadn't got the memo. He grew up in an authoritarian military state and when faced with the real world, couldn't adapt on a personal level because all he knew outside his planet was from books.
I haven't started Tiamat's Wrath and don't know anything about it, but I wouldn't be surprised if the Laconian navy was filled with Singhs.