Oda says that then has grunt marines transporting slave ships, protecting slave auction houses, gassing and capturing civilians for the CDs,etc.
That was 40 years ago. Things changed and the marines have become more and more akin to gangsters.
Our first introduction to them is literally one ordering the murder of a little girl. Then our second intro is one taking bribes from a monster who is holding an island hostage. Even now Oda has gone out of his way to show the marines are no different than pirates.
Sure some people have idolized images of marines but we’re told directly from the manga that the WG puts out Marine propaganda to give people that image of them.
Which is, again, "our" projection on the manga, not Oda's and not what he is writing into the manga. Which we are entitled to have, but that is another matter. In the manga Shanks is portrayed as someone good, to look up to, etc., but I despise him lol.
In the manga, as said, even if you want to erase Harald's recent statement with some motivation (which is not holding up imho, but still), we have the revolutionaries themselves saying the problem are the CDs and not the Marines, or even the WG at large.
At EH there was another statement of this kind by Vegapunk:
And on and on.
So, how many times Oda has to repeat this concept? Harald, Vegapunk, the Revos, and we can go back.
You are perfectly entitled to think marines are evil scumbags, but if we have to analyse the perspective
of the manga, the pov of the story, it's pretty clear - and even explicit - that the Marines are not supposed to be villains or bad
at large, that's why there are mostly good people inside. Again, good people from the perspective of the manga, because we can argue all day how Garp or Sengoku could be bad people, but in the manga they are portrayed as good people.