Well, first off, the Shichibukai are shaky at best because they're simply pirates paid off by the government not to stir trouble, but there's no guarantee because the WG exercises essentially zero authority over them. Crocodile used his position to do anything he wanted, and no one picked up on his facade except Smoker, for example.
As for the marines, I agree with you that the low-ranking guys are all just nice civilians, but I should've specified that I'm talking about the important officers in the Marines, aka the people that are giving the orders. It's not like anything a low-ranking officer will say or feel will have any bearing on the actions of the WG itself. In that sense, Aokiji and Garp in probably in a minority, because of the widespread corruption and heartlessness we've seen in the series. It does make a little sense that all Marines would be trained to despise pirates, but like I mentioned before, they take their own meaning of justice to the extreme.
As for Akainu, IIRC he had something to do with the Water 7 arc, but I forget what exactly; I could be wrong. I do remember that one of the vice-admirals shot one of his men for not obeying orders during the buster call--that's just downright cruel.
I totally agree for the shichibukais, but that doesn't make them "extreme", right ?? They are mere corsairs (or privateers), and those are not extreme conceptually IMHO. It depends on the individual then (Croco, etc.), but it's not this concept which is at fault.
About the marines, don't forget there are 3 admirals only, which means at least 33% are nice (that's not too bad a minority ), and we aren't even sure the 2 others are not nice ! And also, when you see how Garp is allowed to do what he does, it means his superiors can't be that bad (to an extent). I am not saying they are all nice, but considering their situation it's not that bad at all.
As for Aka Inu, I don't remember he was involved during the Water Seven arc ^^
maybe they are not really alowing Garp to do this, just tolerating because he is a hero who fought Gol D. Rogers.Or even maybe they are not controlling him too tightly because he can´t interfir in their gran scheme.
Aside from that i must say that it is not productive to try to see if the admirals are good or bad from soo little info.Even Aokiji who everyone think he is good may change in the future saying that all he did was to use them to get to his goal.