Even in the games where he helps people, he only did it to get something out of it;
That would make him an anti-hero and that's what I'd count him as for almost all his playable appearances with plots.
Wario Land: Steals things from pirates. As he's directly opposed to a group of villains, that probably makes him an anti-hero; he's doing something good, but for the wrong reasons.
Virtual Boy: Tries to steal treasure from some strange creatures who cause him to be swallowed up by the Earth in retaliation and he has to fight his way out. I'd call him an anti-hero here too, though it is less obvious than the first game.
Wario Land 2: Is attacked by the pirate group who instigate the battle this time. As he's going around recovering his possessions which were stolen, one could argue him as a hero in that he doesn't really do anything particularly bad.
Wario Land 3: Trapped in another world, has to go on a quest to return home and winds up saving this new world. Might not have been his intention, but once you've saved a world it gets harder to call you a villain. Also don't think he really did anything bad here.
Wario Land 4: Loots a pyramid. Fairly neutral act as raiding ancient structures is not inherently. Coincidentally saves a princess; anti-hero again.
Shake It: Saves another world in hopes of getting infinite money in reward. He's now officially accomplished far more good than other heroes. Still totally a hero.
Legend of Stafy 3: (EDIT: Not playable and does not belong on list. Leaving it because it's awesome.) Cameo appearances as a parody of Wario Land 4. Helps the main character; perhaps not intentionally, but still, helping heroes against villains does NOT help the argument of him as a villain.
Wario World: One of his stolen treasures was alive and evil. Saved a bunch of Spritelings which were imprisoned by it, making them the third race of creatures that Wario has saved. That he's repeatedly saving the world for the wrong reasons doesn't change the fact that he's the hero of these stories; an anti-hero, but still, a hero.
Warioware: Makes a successful video game company. Tries to steal all the profits for himself, but reluctantly does pay his employees. So he's a legitimate business owner. Greedy, but legitimate.
Mega Party Games: More wacky hi-jinks as he continues to be an inexplicably successful businessman.
Twisted: Breaks his GBA and has Dr. Crygor make him a new one. Crygor makes a motion-controlled system and Wario gets rich by making motion-controlled games. ...There's a joke about the Wii in there somewhere.
*continued*
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In this world which will someday fall to ruin...
...dreams and hopes don't matter at all.