In 'MegaMan X', Zero mentions that Vile "is designed to be a war machine." In 'MegaMan Maverick Hunter X', the manual mentions that Vile has irreparable damage to his electronic brain that makes him psychopathic and violent. So what was Vile built to do? And how did he receive that damage? Was that due to a defect in the process of being manufactured, or was there some kind of accident? Why is he allowed to be a Hunter (and not replaced) if something is known to be wrong with him? As Vile tells Armored Armadillo, "Pity!? Don't pity me! You don't know anything about me!" There is definitely a bigger backstory to Vile that hasn't been fully revealed yet, but there definitely are some clues.
Personally, I take Zero's words literally and assume he was built to be a military Reploid, the 21XX equivalent of a Sniper Joe, rather than a Maverick Hunter. That would also explain why he carries such ridiculously bloated levels of firepower, even in comparison to other Hunters: he's designed to be a one-robot-army. It would further explain why he's not too bothered by loss of human life, because even if his electronic brain was working properly, as a war Reploid he would probably be expected to cause some degree of collateral damage due to the conditions of war. Under this theory, because of his defect Vile was taken off the battlefield, but Sigma may have taken pity on him and believed that he could be rehabilitated somewhat under his supervision and allowed him to join the Maverick Hunters.
This would position Sigma as a sort of father figure to Vile, a dynamic we see play out in 'MegaMan Maverick Hunter X': Vile initially expected Sigma to scold him rather than ask him to join his cause (the former seemingly being a common occurrence for Vile), and Sigma has high expectations of Vile but is subsequently disappointed by Vile's 'misbehavior' in Vile's ending. Additionally, as a Reploid of a type which may have been mass-produced (and a defective model) Vile may therefore harbor his extreme animosity for X because X is unique and "special" whereas according to this theory Vile was not made to be special, being part of a line, and is only "special" in all the wrong ways. Even though Vile seems to be the only of his model among the Hunters, deep down he knows he's just another manufactured Reploid soldier regardless, one who's faulty, expendable, and easily replaced. Vile therefore feels he must constantly prove his worth to himself and the world, something Hunters of his rank usually don't worry about, and rebels against authority (the Hunters and esp. Sigma) like an angry teenager lashing out. It also explains why he seeks Sigma's approval in other instances, like in MegaMan X8; Sigma may be the only person whose respect he may be able to earn. Zero, having been a "maverick" not unlike himself who worked his way up to be an SA-class Hunter, is someone whom Vile probably views most as a rival and the main person with whom Vile competes and to whom he compares himself.
What I like about this theory is it grounds Vile's hatred of X (and to a lesser extent Zero and Sigma) in the harsh truths of his life. X is the child with the silver spoon, whereas Vile was manufactured to be a single unit of a line of war machines and he doesn't even function properly. He's good at blowing things up and stands out in that department, but that's all he's good at. He doesn't even identify with his fellow high-level Hunters; after all, there's only one Chill Penguin, Sting Chameleon, etc.; even Zero, his main rival in the SA-class, is a unique model. The reason for his especially strong dislike of Storm Eagle is that he, like all the other special Hunter models in Vile's view, takes his combat superiority for granted and in Storm Eagle's case moral superiority as well. So of course Vile has the biggest chip on his shoulder; he has the most to prove! It also explains why he's so hung up on ranks; having a high rank reassures his ego that he's better than low-rankers like X and the mass-produced fodder models that make up the bulk of the Hunters. All of a sudden, Vile's behavior, especially in 'MegaMan Maverick Hunter X', makes a ton more sense. Vile wants to "change the world" because he NEEDS to believe he can change the world, because otherwise he's just a defective toy solider to be replaced and forgotten the minute he becomes either too disobedient or worse at blowing stuff up than the new VAVA on the block.
But this is just a theory. What do you all think? What about alternative plausible theories? Do you have additional information that could help to illuminate the truth of Vile's mysterious past?