Those where a traitor's army so its not that surprising they where annihilated to the last man, to make an example out of them.
While the shortcomings of the adaptation are clear so i'm speculating so far.
- As far as we can see, there's no conscripted soldiers on Ferme's army. So all of them are likely Ferme's personal army and/or professional soldiers with a stake on the rebellion. For Bartolo and Alnus leaving them alive has its risks, and killing them all cleanly removes all rebellion roots.
- There's the (very high) risk of them turning to banditry or leaving for another country and revealing the inner turmoil of Rosice Kingdom if not pursued.
So it is understandable than they where all killed.
@humbleoverlord while not clearly stated, i think it is obvious the MC becomes ambitious when he accepts to become Raymond (the king's brother) adopted son and heir. A regular non-ambitious japanese MC would have refused and stuck to farming...