It aint the Marines that are the problem...
It's the World Government they work for...
That's what Oda seems to be going for...
Given what we've seen of the Admirals (their childhood pictures) and interactions when they're not with pirates, we get a completely different impression. Akainu killed civilians because of their apparent threat to stability, Kizaru is just following orders, Kuzan fucked off for what he thought was 'right', Fuji is Bae and Ryokogyu... well, there will always be super bad apples...
Do I think it's a good thing that all 'villains' are portrayed as 'they're not so bad!' ? No, but I feel that's Oda's way.