I don’t think he’s a Moriah-made zombie or anything like that, but a living being made from the bodies of pirates who perished at Marineford created by Vegapunk.
He parallels Kuma and is connected to the replacement of the Warlord system with very strong and easily controlled soldiers.
-In Thriller Bark, we learn from one of the most skilled medical professionals on the planet that even the most skilled doctor cannot raise the dead through any means; if this will ever be contradicted in the story, it would be by Vegapunk surpassing Hogback’s medical skills.
-Vegapunk has already created a dead man walking in the form of Kuma; regardless if the World Government has any methods to ensure any form of eternal life, Vegapunk has been shown to experiment with mortality before the time skip. The World Government has been hinted to some degree as having methods of maintaining youth or extended lifespans, but raising the dead is still something that has yet to occur. Such a thing would be appropriate, however, to reveal at a meeting of the leaders of many countries as a show of force and power in uncertain times.
-The fate of Oars Jr has not been revealed, but it’s clear that Moriah did not manage to claim the corpse. I believe that Weevil’s enormous strength, and many physical similarities to Oars Jr, is sourced from being partially made from the giant himself along with other available materials, such as Jozu’s defrosted arm. However, it is unlikely that he was made from any parts of Whitebeard, as his corpse was protected by Shanks; as such, Weevil believes he is just a son of Whitebeard, as apposed to literally being Whitebeard.
-The reason for his odd behavior and his sudden appearance in the world is because he is in fact very young, having been created only after Marineford. Bakkin is basically his handler and the true Shichibukai, having known Whitebeard in the past enough to act as a former lover, and convinced Weevil to steer away from attacking Blackbeard in favor of continuing the mopping up of former Whitebeard affiliates, which is a goal that favors the World Government more than anything else.
-Weevil is a expression of the proposed replacement for the Warlord system; the World Government programs to animate the bodies of dead enemies, resulting in extremely strong and easily controlled human weapons. Weevil is the prototype for such individuals no different than how Kuma is the prototype for the Pacifistas. Weevil was created specifically to humiliate those that would stand against the World Government, possibly by the Five Elders, with the Marines having no involvement except for Fujitora.
-Weevil has a drive to find Whitebeard’s ‘treasure’, which is in fact his home island, and desires to eliminate the rest of Whitebeard Pirates and their subordinates out of a misplaced desire to be one of Whitebeard’s ‘sons’. Weevil will likely realize what he actually is when he arrives to Whitebeard’s home island. Weevil will either destroy himself out of penance or will attempt to continue Whitebeard’s legacy in some way that will likely lead him to fight Blackbeard as a living representative of Whitebeard’s crew, possibly ‘raised’ by Marco into a honorable man once Bakkin is out of the picture.
*Theory by Pokonic