Republic Book 1 Section 2

Cephalus' son Polemarchus is not as old and thus not as crafty as his old man. He thinks he can trick Socrates into believing that the temples' obedience is in fact justice. Socrates is older than he is, so he fails.

Heh. Remember when men became more capable as they got older? What a blast from the past. Good times.

 

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"Socrates continues, it is a given that the possibility exists that our friends may be in fact bad, or unjust, men; and it can be that our enemies may be good men"

Unfortunately, Plato was highly influential. Christianity in particular picked this up and ran hard. 

Socrates isn't wrong. Indeed this is exactly why the Athenian Assembly hated him so much; they were bad men, and he was not. 

However, it is still true that one ought to help your friends and harm your enemies.

Nobody wakes up in the morning, gets a cup of coffee, and plots how to be most evil. It simply isn't prudent. There is no god of evil handing out boons for racking up evil points; being evil on purpose is all cost and no benefit. (Usual disclaimer; when I say 'evil' it's a metaphorical shorthand; you know what I mean. No such thing inherently exists.)

Humans are evil because they don't care about being good or about the heavens. Instrumentally evil, not teleologically evil. [Being good] refers to their own long-term self-interest. They don't care because they're stupid and irrational; Socrates was right about that too.

 As a result, you have two options:
1. Doesn't care whether your enemies are bad or not
2. Does care whether your enemies are bad; meaning they care whether they themselves are bad.

Genuinely bad men aren't ever going to stop. They are presumptively self-absorbed in the usual narcissistic way. If you have an argument against helping your friends and punishing your enemies, the only folk you will restrain are the particularly ethical, who really should be helping their typically-ethical friends and harming their typically-unethical enemies.

The real rule: don't cooperate with defectors. It doesn't matter if they're bad folk badding on you for the bads, or if they're good folk who are mistaken, or if good folk badding on you because you're bad and deserve it. If they're defecting on you, defect on them. Cooperating with defectors is super gross even if you're a criminal and deserve to be defected upon. If you're going to break the law, at least commit to it properly.

Likewise, cooperate with cooperators. This is always rational, because you will always gain more from future cooperation than you will by defecting now and having no cooperation in the future. If bad men cooperate, are they really bad? (P.S. The fact evil can't exist should not be some mystery.)

That said, some humans are [evil] gods, handing out evil points to themselves. Let me show you how this is equivalent to the above.
Some humans - something like 80% of modern Fascists - value only defection. If they can't be a deviant - lying, cheating, and ideally killing their way through life - they might as well just kill themselves, because there's just no point. Doing inherently evil acts is the only thing they find rewarding. 

However, they don't find it rewarding because it's evil, they find it rewarding due to a stupid genetic algorithm which indicates the effective defector has hero-class status. They want heroically high status, and can't "see" any indications of this except the crime indicator light. If they can't be a hero, they think they might as well be dead. Worse, this is true; I'm talking about spiteful mutants here. We all would be better off if they were dead or, ideally, had never been born at all.

Occasional exception: some grass monkeys are born to harass and ideally kill outsiders. Genetic raiders. In huge transnational empires, there are nowhere near enough outsiders for them to vent their urges on. Unless videogames can fool their instincts, they will become severely twisted up inside. 

Note that even if there were an evil god handing out evil gifts in exchange for evil points, it still wouldn't be prudent. Either you would pay the victims because the boons were worth more than the wergeld, in which case it's a normal cooperative merchant purchase, or the victims would, on average, cause more damage to you fighting back than you gained in boons. (P.S. The fact evil can't exist as distinct from stupidity should not be some mystery.)

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"But again, Socrates demurs: He argues that returning evil for evil does not constitute justice. Analogically, he argues that if we harm a horse, we make that horse a worse horse; if we harm a dog, we simply achieve a worse dog."

I hope Plato is putting words in Socrates' mouth, or he's playing devil's advocate, as Diogenes was prone to doing. He sounds like an idiot here.

The whole point of the thing called [morality] is the tension between the long-term and the short-term. If we harm a bad dog, in extremis we have a dead dog, meaning less bad in the world and thus a less bad world. In fact: better dead than red. Ideally we apply a small harm to a generally good dog, which discourages them from being bad and we don't have to actually kill them. We pay costs in exchange for a worse dog, yes: only in the short term.

Of course Christianity was on Plato's nonsense like white on rice. In their defence, peasants are like children and can't understand the long term. They can see only deontology-like rules and the immediate social consequences of breaking them. Setting the immediate-term rules such that the long-term is satisfied is a task for the peasant's betters, one which peasants refuse to understand even if you explain it to them. Whips optional; it doesn't matter.

In other words, we always need to harm the bad in the dog.  Always kill the bad. If the dog has so much bad that harming its bad is lethal to the whole, then we had a bad dog and we're better off without it. Usually even the bad dog itself is better off not existing, as regardless they'll be punished repeatedly by Gnon until they die.

"Thus Socrates argues that we cannot achieve justice by doing evil to men who are already evil, and unjust. And Polemarchus concurs with this conclusion."

Let's de-Sophize. Rectified terms: Socrates argues that defection cannot be removed by removing the defectors, and Polemarchus agrees. Pure Satanism.