I think, he was indeed actually surprised about their strength and the resulting unison raid. Not in a way of thinking, that they might be really dangerous to him. No, I think, he was just surprised, because in the beginning, he thought of all FT members as Makarov's weak children. Yet his seven kins and Bluenote were disposed of in the end. And now he witnesses first-hand, why that could happen. So it was just some kind of an acknowledgement IMHO.
Something about my own Grimoire Law question just slipped my mind. Hades only used it to counter Makarov's Fairy Law. IIRC, he (as Purehito) was the inventor of FL and the one, who said something along the lines, that FL was meant to discourage the enemy. And that it should not just be used, whenever the caster feels like it (correct me, if I'm wrong here).
My point is: He didn't use it (yet?). This might just be his big confidence. But maybe he did still retain some of his 'former honor' afterall? I mean, he kind of apologized to Makarov for all this, too... To sum it up, maybe Hades is not a really bad person deep down and there will be some redeem for him (ref. Darth Vader ^^).
This is not meant as a prediction or anything (too far-fetched for that). I'm just curious, what you all think about this or if there are any further pros/cons