i) Is it cheating to cheat a cheater? Suppose I'm in a poker game. My opponent has bribed the dealer to stack the deck in his favor. If I cheat just enough to compensate for the cardsharp, is that dishonest?
I'm not cheating to secure unfair advantage. To the contrary, I'm taking countermeasures to restore the balance. Make an unfair situation fair again. Call that counter-cheating, to rectify the disparity.
Indeed, the very concept of cheating presumes a situation in which most folks play by the rules. If nobody plays by the rules, then there are no de facto rules.
In fact, even if I cheated more than necessary, there's a sense in which my opponent would have it coming. Like double restitution for theft (cf. Exod 22; Lev 6). If a thief only has to repay what he stole, he has no incentive to refrain from stealing. He's only returning stolen property because he got caught on that occasion. Consider all the other times he got away with it. So unless there's an additional sanction to deter him, he has nothing to lose and everything to gain by continuing to steal.
ii) Many Christians avoid these dicey issues. They cast themselves as "absolutists." They think that simplifies matters. Just do the right thing and let God sort out the results.
Now, I myself am an absolutist in the sense that I believe in moral absolutes. Some actions are intrinsically right or wrong, obligatory or prohibitory. However, belief in moral absolutes doesn't entail that every action is reducible to absolute duties, without regard to circumstances or consequences.
Moreover, "absolutism" is deceptively simple. For instance, if I do nothing to offset the actions of an evildoer, doesn't that make me complicit in his evil? If nothing is done to overcome his evil actions, then I'm passively facilitating his evil. So that doesn't ipso facto get me off the hook.
Now the point of this post is not to assess the ethics of counter-cheating at poker. Poker is just a game. A social convention. I merely use that as a convenient example to illustrate morally serious situations (unlike poker) where prima facie obligations may overridden by higher obligations.