From what I can ascertain, and I am by no means an expert, the Swords are considered mostly "bad" cards because of the Golden Dawn attributions of the cards to the Tree of Life, present in both the RWS and the Thoth (the Marseilles predates these and so is open to a more liberal interpretation). There are some mitigating astrological attributions, but roughly (very roughly) it goes kind of like this:
The four suits represent Mind (Swords, Air), Body (Discs, Earth), Will (Wands, Fire) and Emotion (Cups, Water). Each of them descends, as it were, from the top of the Tree, Keter, which is pure spirit, the potentiality of each element of the suit, represented by the Ace, down towards Malkuth, which is the pure material aspect of the suit, represented by the Tens. The farther down you go down the Tree and away from the ace, the suit is "polluted" with other forces. In suits such as Discs, this is harmonious, since the Earth is happiest when it is at the bottom; this is why the Ten of Discs, for example, is such a good card. Cups and Wands are (very roughly) "medium" suits of elements, so they can go either way, and so they do; there are both good and bad cards there. The Tens are usually "bad" cards since they have become so polluted they lose the coherency of their suits, and descend to the next. In the case of the Ten of Discs, however, there is also nowhere to descend to, it is the bottom of the bottom, a very "earthy" position, from which there is no place to go but up from there.
However, Swords, representing the mind, spirit, thought are happiest when in Keter; they are pure energy of consciousness, and so very much "dislike" going down into the earth, since it against the nature of pure thought to be forced into material aspects. Illustrating this is, for example, is the Three of Swords; the three swords in the heart show pure thought wounding emotion (in the Thoth this card is similar, only with a white flower instead of a heart.) showing that too much thinking and too little emotion can have devastating consequences. Of course, like I said, they're not all bad, but then the thing that mitigates this unharmonious balance is the astrological attribution of the card. Here's a rough diagram of the Tree of Life, to understand better. What's important in this case, is simply, that it has a top, and a bottom.
http://janetboyer.typepad.com/.a/6a013485f24774970c0133f4f8ae05970b-800wi
I'm not sure I explained all this as well as others could, so I hope you get what I was trying to say