Should I touch on the aftermath of the battle, and the mass murder, rape, and enslavement of the army's 12,000 camp followers?
Yes, you should. If this is something that happened in your world, you cannot just ignore it or gloss over it. This event is too big to be swept under the carpet. Considering your setting, it would have some impact on the characters. If something has impact on your characters, you cannot just erase it - that would change the characters.
You are saying that you do not want this event to overshadow the rest of your story. In that case, don't give it a lot of space - give it some. Don't give a first-hand account of a woman being raped, but put some distance between the protagonist and the events.
Here's an example of how such a situation can be treated, from The Grace of Kings by Ken Liu, the protagonist conquering a city:
It was whispered among the surrendered Imperial troops that they would be allowed to loot Pan as a reward for their submission to Duke Garu - as long as they did not kill anyone A few bold soldiers went into the streets to test out the rumour. Kuni's men watched them but did nothing. By afternoon, the former Imperial barracks were empty.
The soldiers had free rein of the entire city. Pan was treated as though it had been conquered, except that the conquering army was composed of the men who had sworn to defend it. They broke into the wealthy mansions lining the streets, took whatever they fancied, and did as they liked to the men and women they found inside them - the soldiers did take care not to kill anyone, but there were many forms of suffering short of death.
For ten days the streets of Pan became a living hell, and families huddled in basements and shuddered while they listened to the cries and screams of the less fortunate. The Immaculate City became stained with terror, blood, avarice and cravenness.
(Ken Liu, The Grace of Kings, chapter 30 - Master of Pan)
In this example, the protagonist is sufficiently far from the action, we do not observe the horrors first-hand. But the horrors do happen, and have consequences. The protagonist is of course not a barbarian who "does nothing but rape and slaughter", he's a relatively good guy, but his hands are not clean either.