He probably think's the Wg is a corrupted organisation, that's why he made revolutionaries that try's to overthrow the WG, he probably had a many chances to see that WG is just a pile of shit.
He saw the WG as the corrupted piece of sh*t that it is but Garp couldn't see that. After what happened to Ace and somewhere around the timeskip i think Garp also realized the same thing. That's why he left the marines.
I imagine he was pulled into the marines by Garp and was given a pretty high rank
Big enough to actually see all the behind the scenes stuff and thus realised what WG rly was
He then left and started the revolutionaries to overthrow the WG.
Also cause he learned all the behind the scenes stuff he has alot of info on WG and so eh can cause real damage to them thus is their top priority