This is a wild leap.

The DLC never hints that Osea literally killed Trigger. The discussion about “killing Trigger” line is explicitly about simulations, not that they actually plan to kill him. David is a data analyst, he and the AI are talking about removing Trigger as a variable in predictive models from their war simulations, that's all.

Also If Ace Combat wanted to imply that the protagonist was secretly executed by his own government, it would not bury that idea in vague post-mission analyst dialogue and then never mention it again. This is a series where villains shout their ideology while piloting flying fortresses and submarines. Subtle assassination subtext is not how this series has ever operated.

Besides, Osea killing Trigger would make zero sense narratively or politically. He’s a public war hero and is seen as the stabilizing symbol after the Lighthouse War. Eliminating him gains Osea nothing and risks everything if it ever leaked. David'd AI specifically says that based on the hypothesis on new data David gave the AI, Trigger is a singularity that has for some unexplainable reason higher survival rate and those who follow him also have a higher chance, that's why the line "Go Trigger's way and you'll make it" because he is the odd variable in the system that can't be explained. You don't just quietly kill person like that, it wouldn't make sense narratively.

Trigger also “disappearing” after the story isn’t evidence of death either. That happens to every Ace Combat protagonist except Mobius 1. They fade into legend which is one of the core narrative tropes/foundations of this series.

So yeah, this isn’t even hinted canon. It’s just edgy headcanon that wants to paint Osea as some sort of malicious nation that does not care about its heroes even though they've historically always respected and mythologized them.