Of course it was vengeance. Swallwell is a traitor, who had a sexual relationship with a known Chinese spy (who seems to have gotten around a little, btw), compromising himself in the process.

He got had, and he was too naive and puppy-lovesick to see it.

So yes. Vengeance for his stupidity and lack of care. The fact that Swallwell is whining about it in this way, proves that there are some maturity issues there.