Aokiji being an Admiral he had access to highly classified information and in the movie Z it showed how dangerous these informations could be in the hands of an outlaw. The Dyna Stones were just one but Aokiji would know more…
Aokiji simply resigned from the Marines meaning they couldn’t place a bounty on his head but now he allied with the Blackbeard Pirates so this means that the World Govement couldn’t just let the things that he know fall into the wrong hands, so Aokiji would have a bounty on his head.
Bounties are based on how dangerous you are. Aokiji didn’t do anything yet but the informations he possesses are highly classified so he needs to be captured as soon as possible.
Possible bounty would be far beyond 1 Billion Berries since no one is stupid enough to attack an ex admiral for 100m, that’s just not worth it!
His real motive after his resignation from the Marines
Aokiji’s plan wouldn’t be evading the Marines or becoming a notorious pirate, his plan would be to overthrow Akainu since his policy of Absolute Justice is something that would move the Marines to a wrong path.
And Aokiji was a passionate marine believing in what’s right unlike Akainu with his unforgiving attitude.
Aokiji vs Akainu
Akainu didn’t let Aokiji freely leave after the battle. I believe that the fight ended with Akainu not being able to walk out neither was Aokiji but Akainu was declared the winner because after 10 days he was the strongest one but if Akainu was to kill his fellow comrade there he would be labeled as a murderer so his position would be left in jeopardy.
Now Aokiji could only do this if Akainu is overthrown, dead or demoted (now the Marineford arc would proved that’s impossible to do any of these things). So Aokiji have faith that Luffy will fight Akainu in the future and this will lead to Akainu dying or being demoted. And the Gorosei will not allow someone of Akainu’s level to be defeated that would make the navy weak so they would need a new Fleet Admiral.
How will Aokiji get his position in the Marines back??
He is currently an ally of the Blackbeard Pirates meaning he would know a lot about the crew so Aokiji will aid Luffy in his fight with Blackbeard only by the condition that he would get to deliver the Blackbeard Pirates to the Gorosei thus getting his original position as the new Fleet Admiral.
Future role in the Marine after he becomes Fleet Admiral
This would leave Aokiji in the same position as Akainu having to answer directly to the Gorosei so this is the part in this theory that makes Aokiji’s plan complete.
A Deal with the Revolutionaries
Ok now he is allied with the Blackbeard Pirates but he could have convinced Dragon to overthrow the World Government after he becomes Fleet Admiral. As soon as Dragon makes his move the World Government would make the Marines protect them but now the command would be with Aokiji so he has the ability to deny the direct oder leaving the World Government powerless and right for the taking by the Revolutionaries.
*Theory by King of beasts kaido