*spoler!!* Why did Japanese officer slapped Nagase?

The only thing I can come up with is that this is the point at which Nagase began to realize that what he was told by his superiors was lies and what Lomax told him was truth.

In the Imperial Japanese Army, superior officers were pretty brutal to their inferiors. He was angry because Nagase wasn't translating as he was supposed to. He was just talking to Lomax independently without telling the officer what Lomax was saying. The officer couldn't understand Lomax but could tell that he was saying important things, not only from the way Lomax was talking but from how distressed Nagase was by it. And he was angry that Nagase wasn't telling him what was going on.

I'm watching the scene again now and will translate.

First the officer grabs Nagase and pulls him back, and while looking down at Lomax, says "Koitsu wa ima nan to itta ka?", which means "What did he say just now?"

Nagase hesitates and doesn't say anything, perhaps afraid of telling the officer the distressing things Lomax had just said, and therefore he just essentially defied his superior officer.

The officer gets angrier and looks at Nagase, and while shoving him, says "Kisama, koitsu wa ima nan to itta ka?!?" "You bastard, what did he just say?!?"

Nagase still hesitates and doesn't answer, and the officer begins hitting him, and the flashback ends.

Evidently this is where Nagase begins having doubts and questioning everything he knows.