Our main sources for Socrates are Plato's dialogues, so it's hard to separate Socrates from Plato. But I believe Plato is known more for the metaphysics and Socrates for the Socratic mode of questioning.

Socrates was a gadfly who interrogated folks to show that conventional wisdom is incoherent or otherwise muddled. The irony was that Socrates was wisest for being humble, for not presuming that his mere opinions are the same as justified true beliefs (knowledge). Dialectically, Plato wrote dialogues perhaps to honour Socrates or to highlight the importance of the oral tradition for philosophy. I'm not an expert on ancient philosophy, though...

Part of the meaning of the devil's having his own reasons is that the Bible is ambiguous or contradictory enough to allow for all possible interpretations, including a demonic one.