Earlier quoted context omitted.
Can we agree that “someone would have done this anyway” is a poor justification for one’s actions? That’s what you say when you 1) recognize that you did a bad thing, but 2) don’t want to be blamed for it.
I actually don't agree with this. Assume that the following premises are true: * action x is legal, but results in a worse world for everyone. * doing action x benefits the first 1000 people who do it. * much more than 1000 are capable of doing x, including me. * if I do it now, I will be one of the first 1000. If these are true, I think one should do action x. I believe one would be completely justified in doing it.
I think the answer is inherent to the question of whether it's "justified", and the fact that you're being downvoted.
If Prisoner's Dilemma is played once, the optimal strategy is to defect. If it's repeated an unknown number of times, the optimal strategy is tit-for-tat.
Now, if you're an identifiable actor in a pool of tit-for-tat-players, and you have a history of defecting first, you're going to face opponents who normally play tit-for-tat start to defect against you first, too. In the end, you will end up as one of the worst peformers in the player pool.
So when you're asking if it is "justifiable" to defect in a game like this, it is the same as asking "will I be treated worse if I defect in this game". The downvote confirms this, I think. It means that in this population pool, bad behavior is punished (or people pretend it is)
Now, if defecting can either be done secretly or if you're in a popluation pool where everyone defects anyway, then always-defect is probably the nash equillibrium. In that case, defecting will be seen as justifiable.