What would the incentive to become a traitor before the battle of Chapter 33 be? Before Quirrell added the ability to switch sides, you'd just be helping your army (which you've already developed a bond with in the first battle), and therefore yourself, lose. I'd expect this to strongly outweigh the fun of being a spy.

I just Googled for "airsoft betrayal" and "paintball betrayal." I found no stories of similar events in either sport. (I did however find one person hypothetically talking about betrayal in laser tag, even though many/most systems ignore friendly fire.)