If the player has a second account on the other faction, then they would be violating the Terms of Service.
Cheating: Don't do it. Play fair. Only use official Niantic Labs software and remember Ingress is meant to be played on a mobile device and get you outside to explore your world! Methods of cheating, unfortunately, are limited only by cheaters' imaginations, but include at a minimum the following: using modified or unofficial software, playing with multiple accounts (one account per player, please); sharing accounts; win trading; using tools or techniques to alter or falsify your location; or selling or trading accounts. If you suspect someone of cheating, don't call them out in Comm or demand they show their face, just report it via the Help Center and focus on your mission.
However, if it is their only account, there are no rules to prevent them from playing how they want to play. There's also no way to verify that a user's sole purpose is to "watch the world burn" unless they explicitly say so (and even that could be sarcasm).
Still, feel free to report any players that you feel may be double agents. Chances are they have a second account and if you can raise enough suspicion, Niantic may investigate your case.