Meowth has been a staple of Team Rocket since the beginning of the Pokémon anime, with his fast-talking, quick-scheming nature and gleeful pursuits of villainy alongside his teammates, no one could argue that the Scratch Cat Pokémon was exactly... put together. But the moment you realize that unlike the other Pokémon trained by Jessie and James, Meowth is, for all intents and purposes, a wild Pokémon, everything changes.
In Season 1, Episode 17, "Island of the Giant Pokémon," Team Rocket's Ekans and Koffing reveal that were only fighting out of loyalty to Jessie and James. As such, when Meowth orders them to attack and capture Pikachu, they refuse, demonstrating that only Meowth is actually a part of Team Rocket because he wants to be. While other Pokémon would join Jessie and James of their own accord, they are indeed joining Jessie and James -- not necessarily Team Rocket and the goals of the organization.
This means that Meowth is not only an equal to Jessie and James, but is fully responsible for every evil action and scheme of his. Stealing Pokémon, causing harm to others -- they're all things he has done of his own accord, without the coercing force of a Pokéball or Trainer. Meowth is actually...evil. Fortunately, it's the laughable, ineffective type of evil that has endeared his character to the masses with his petty selfishness, guilt-less thefts and habit of falling in love with every kitten-like female Pokémon he meets -- and yet it was that very thing that truly started him on the path to who he is today.
Season 2, Episode 16, "Go West Young Meowth," delves deep into our talking cat's backstory, explaining why he can talk like humans and walk on two feet -- a trait that, while it would become commonplace for Meowths in later series, was not how they were originally designed, with most Meowths in the episode being quadrupedal.
Orphaned as a kitten with no memories of his family, Meowth wandered the land looking for food until one day he tipped over a basket of baseballs, inadvertently trapping himself. Tied up and strung from a tree, he ended up with a prime viewing spot for a movie hours later that showed a Meowth being fed generously by humans in Hollywood. So he escaped his confinement and traveled to the city.
City living wasn't quite as he had imagined, and Meowth was forced to steal food, eventually joining a gang led by a Persian. But while these acts could be seen as "evil," at this point, Meowth and his cohorts were nothing more than wild Pokémon trying to survive. Unless someone caught or adopted them, they would have no way of making money to buy the food, even if they had wanted to.
But everything changed when Meowzie stepped on the scene -- the beautiful she-cat immediately caught Meowth's attention and heart. But the prim and proper show-cat adored humans, who adored her, not some lame dirty grubby thief from the streets. Meowth took this to heart -- and decided that in order to win the love of his life, he had to change. He had to become... human.
He learned to walk on two legs and learned to speak, his first word being "rocket," a development that would later lead him to join Team Rocket after Meowzie's ruthless rejection. Now, in addition to being penniless, she considered him a freak, and so he joined Team Rocket, seeking fame, fortune and the power to win Meowzie's love.
Thus, Meowth became a cat who's always looking for an angle, a means by which to get ahead, without being pushed down himself. His great lack of food at the beginning of his life inspired his heavy appetite and greed, with some of the most vicious fights between the Rocket trio occurring over their meager meals. Add to this the fact that, in many ways, Meowth is very much his own trainer, having taught himself the skills he now uses daily, and it's impossible to see him willingly serving anyone else (except maybe Giovanni).
While the majority of Pokémon are portrayed as friends at best and wild animals at worst, Meowth takes things one step further by exemplifying every possible undesirable trait associated with evil with his wanton greed, lying and constant scheming. But, even as messed up as that is, he does have one saving grace: his loyalty. While Meowth's independent nature means he won't allow others to bully him, his friendship with Jessie and James shows that even at the heart of all his acting out and "evilness," he's still just a cat looking for a little love, affection... and maybe some fried chicken.