You can ask if it's a good idea to make the Golden Ending DLC. And a path where Kamui joins the faction the monsters belong to wouldn't automatically be tightening his own noose, as he calls it. We don't know what the monsters are out for, both kingdoms could have their share of turncoats, and we don't know about any civilizations besides the two kingdoms.
You could frame it as:
''Kamui is disgruntled with both kingdoms and so embraces his draconic side, leading the forces of monsterland against all who oppose them''
Another is:
''The two kingdoms need to be destroyed to end the bloody era. Too much of Hoshido's leadership is spiteful to Nohr; throwing the kingdom's agricultural weight around. Nohr is on a Lebensraum hunting spree, trampling everybody in the way. The monsters are a counter against both kingdoms, both being labeled unfit to lead the next era. Kamui is convinced to join the monster faction, with turncoats and citizens of lands hostile to both kingdoms coming along.''