Misa Amane is a bigger villain than Light, and more ethically corrupt than Light, L, Near and Mello.
I should say now that I do not think that Misa is weak-willed or spineless. I've seen her portrayed that way all the time in fanfiction but I think she's quite the contrary. She's extremely assertive (almost frighteningly so!) and I would even go so far as to say that she's selfish. She's extremely "Misa wants, Misa gets" in how she acts - and she DOES end up getting a lot of what she wants. She pressures Light into a relationship by threatening to kill his other girls and of course uses Rem's love for her as a pawn too.
The most important thing to note, and we've all noticed it - Misa freely gives up her individuality for Light's sake. Everything she does, she does for him. It is true that Misa believes that she owes something to Kira for killing her parents' murderer. But she takes this above and beyond. Once she sees him she is "in love", and therefore I'm inclined to assume that she's acting out of lust as much as admiration. She idealizes him and throws away everything else. Therefore she's putting her own dignity and humanity aside in favor of a boy. She even said that she was okay with being a pawn if she was doing it for him.
And on that note, like the 4 Geniuses, she is manipulative to boot. She's not as smart as they are, but she's not completely stupid either. First off, she clearly takes advantage of Rem. Rem is as innocent as a Shinigami could be in unconditionally loving the girl. But she is constantly spurned by Misa. At the very start of Misa's introduction, Rem tells Misa how a Shinigami dies, after making her promise not to tell. In Misa's first meeting with Light, she reveals the secret. Misa uses Rem even when her memory is wiped, to kill a man (an innocent man, if I remember right) in order to convince Higuchi that she's Kira. Rem ends up dying for Misa, and killing Watari and L along with herself. Granted there is a time gap between this scene and then part 2, but it bothered me always that we never saw her mourning Rem at all.
She completely disregards her friends. At the very start, on her first meeting with Light, she offers to kill her friend who helped her make the Kira tapes. And I think she would have done it without a second glance. Later on, it royally pissed me off when she was perfectly alright with killing L, too. She didn't even think about what it meant, it was simply that she wanted to remember his name for Light.
I would also like to mention that it's possible that Misa is responsible for more deaths than Light. I'm not certain about this, but I believe that Misa was the one with ownership of a Death Note and killing people from the time L died to Light's death. It's true that she's Light's pawn, but she was the one who did it. She also came up with the plan to kill all of those innocent people on TV to prove her power, and Light even said this was dragging down Kira's image.
But the most important thing that separates her from Light (and L) and brands her as awful for me is her intentions are NOT noble. Light and L both act believing that they are Justice and that the things they do, even if they are bad, will better the world in the end. Misa does not act for this reason. That means that everybody that she's killed and all the things that she's done are simply for her own selfish purposes (getting rewards: attention and approval from Light). In this sense she is much more comparable to Higuchi than anyone else.
I think Misa is a symbol for the mindless followers of Kira (or any other cause, really). I think she's enough like the Kira's cult people that attacked Near's building (claiming to follow Kira's beliefs) but abandon it when money falls from the sky. She idealizing a very human and not perfect Light Yagami, and even though she knows this it's possible she chooses not to see his flaws, or just doesn't care. And if there is a true villain in Death Note, it's this kind of ignorance in my opinion. She chooses to latch onto an idea with a closed mind and has no desire to expand or learn, and believes this idea passionately enough to throw her morals and ethics out the window (assuming she had any).
Alright, there we have it. Anyone have any thoughts one way or another?