Ooh, ooh, me, me!
If there's anything that would make Dark Heresy worthwhile, it would be playing a 14 year old Commissar that tries to fit in as an adult and as a child. But he's too hardened by training, and too calloused by his actions to be able to truly be a child. On the cusp of sexual and emotional maturity, he's completely unable to be an adult either. No woman would want a child, and no man would drink with one. He's left bitter and alone as he shuns his childish things and goes through the motions of adulthood, not entirely sure what they mean.
He'd be incredibly pained by it, bottling up his emotions and leaving himself with nothing but diligence and duty with little room for mercy. Unable to be an adult, when given a group of children only a little younger than himself to train, he forces his ways on them, trying to start adult relationships. Drinking, smoking, having sex during the brief hours of relaxation before he has to put a bullet in the head of something a child shouldn't have to kill. I wonder, are there narcotics in 40k? I imagine he'd use them. Engaging in vices because that's what adults do while he's already looking at his gun and wondering if the early retirement package is also what adults do.
He'd be abusive towards his charges. Not murderous, but abusive. He'd pistol whip them for insubordination, and kick them in the ribs when they're down, telling them to do as he says. Not unusual for a Commissar I'm sure, but unusual for a child. It might even degrade into downright sadism at some points. Inflicting physical trauma on his subordinates to release the emotional trauma that he feels.
The cold, bastard shell of an adult protecting and harming the tender, frightened center of a child.
On another note, I want to do something like Band of Brothers with children...