The thing is, I don't character bash often myself, but there's been a few chars I've hated enough to appreciate the feeling. And I'm in defense of any fan activity that doesn't hurt people. Char bashing in fanfic is usually OOC, and can ruin my enjoyment of an otherwise fine story, but hey, if it's what you enjoy doing, knock yourself out. The characters are fictional; they can take it! But even beyond this, I think that character bashing is a perfectly reasonable, understandable form of fanning.
One trouble with understanding bashing is a difference in temperament. People are negative or positive by nature. Some people prefer to praise their favorite movie and some people have more fun reviling their most hated flick. Many a film critic has taken an unholy glee in poetically shredding the likes of Battlefield Earth or Highlander: Endgame. It's a harmless way to channel aggression, and while it can be a little juvenile, most aggression channels tend to be! But if you never get off on a bit of spleen-venting, then character bashing is never going to make sense, anymore than fandom_wank, TWoP, fanficrants, or any other elements of fandom's darkside.
But there's another reason for character hatred, beyond the joy of vitriol. Fandom - all fandom - is fundamentally art criticism taken to new levels. What makes fandom different is that fans, by and large, interrogate a text from within the text. The heart of fandom is fanfic, fanvids, fanart, etc; the communities around these; and of course fan discussion. And fan-discussion does include series-meta: "The Star Trek screenwriters? Were smoking some serious crack when they decided to remove Spock's brain," but is just as often series-internal: "How did Dr. McCoy manage to design that Spock-remote control? Did he minor in neurological bionics?"
Character hatred and bashing takes place within the text. Observe:
"I hate Lana Lang. She's a breathy-voiced, self-absorbed, two-timing little bitch who obviously got the meteor mutant power of making anyone fall hopelessly in love with her, because there's no way all those men and women would adore her otherwise, seeing as she's, as aforementioned, a breathy-voiced, self-absorbed, two-timing little bitch."
"I hate Lana Lang. She's a poorly written character suffering from too much 'telling' and not enough 'showing' of her admirable traits, who is required to take the role of the love interest in too many different storylines to really be feasible. And the actress and/or director makes some unfortunate choices in regards to her diction."
Here's the thing. These two critiques are, functionally, identical. One of them is criticizing from inside the fictional construct, and one is criticizing from without, but ultimately, they're making the same points, expressing the same opinions about a character. Why should one method be considered superior to the other?
In fact, the first method is more satisfying in a couple ways. To begin with, as previously mentioned, it's fun to be mean. And char bashing is a sub-fandom, a fandom-within-fandom: you do it to find other people who agree with you, who go "Right on!" and validate your opinion. But it's also a solution to a problem. The second critique is illuminating flaws in the text. And that's uncomfortable ground for a fan. To a certain extent, yes, we like to poke fun of our fandoms and all that's wrong with them (Star Wars fans hate Star Wars. It's funny because it's so damn true.) But there's a point it can go too far; a point in which too many flaws are illuminated and your love for a piece of fiction falls apart. You don't want to harp for too long about how lousy the writers are, because eventually you'll ask yourself if they do so much wrong, are they really doing anything right? Why am I watching this show anyway?
Bashing the character, rather than the writing of the character, is an answer to this dilemma. In fact, character hatred is if anything a compliment to the writers, that they can inspire such spirited emotion; it's utterly buying into the constructed realm, relating to fictional chars as one relates to real people. Not to mention, bashing is more exciting, more immediate, than a dry, removed, critical analysis; it allows for more opportunities for humor (I've giggled at char-bashing icons even for chars I like) and it's also useful shorthand. Not everyone enjoys taking the time to construct a logical argument; shouting, "Lana Lang sucks!" is more cathartic than, "Smallville's screenwriters need some elementary lessons on believably developing female characters beyond being the focal point of adolescent crushes!" (or even, "Smallville screenwriters have to figure out how to write heterosexual romance as convincing and appealing as the homoerotic subtext, or else should just subvert the now-boring heteronormative cliche and make that subtext text" - because yes, that is a legitimate criticism, even if it's more commonly worded as, "CLana SUX, Clex 4-ever!!!11!!1one!!!!") Character bashing is a valid way for fans to express their opinions - and why else are any of us here in fandom, if not for that?