The main reason was that Dustin detested traitors, and backstabbing individuals deserved harsh punishment.
After dealing with the five rebels, the remaining generals who had sided with the rebellion faced various consequences. Depending on whether they had voluntarily participated or were coerced, they were either demoted, assimilated into the monarchy's forces, or sent to prison.
In the course of addressing the rebellion, Dustin also tasked Albert with leading a unit of the Draken Guardians