Stay away from gossip
Even if your target is so vile as to deserve what you're doing to her, you don't want to get involved. You never really know who you can trust, which is why you should maintain a neutral position at all times. You can rest assured that you'll be pressured to join in on the bashing. It's at that point that you should politely decline.
This is how it works: People have a herd mentality. If you're not with them in talking smack about Betty behind her back, surely you must be against them, making you a threat.
What you have to do is say -- nicely but firmly -- that you don't want to take sides. You want to keep it professional at all times. But the trick is to do this from the get-go. As soon as you start gossiping, it'll be expected of you. Just be sure not to take others' bait.
Unfortunately, it seems the workplace is seriously marred by politics and drama these days. It's enough of a reason for many people to resign and find work elsewhere. I hate to say this, but this problem is characteristic of most if not all companies these days. No firm is immune to this sort of behavior.
Your job is to stay focused on your work and not jump in the gossip fray. If you continue to feel pressured, a conversation with someone in management is in order. However, this often comes with mixed results -- many people on the front lines are themselves the worst offenders when it comes to gossiping.
It's just one of the many tradeoffs to the business world and, more broadly, social relationships in general. Most people have an innate tendency to comment on others' looks, habits, and so on -- it's in our DNA to talk about others, sometimes in the most unflattering ways.
That doesn't mean you have to stoop to their level. Spreading rumors and speaking ill of someone behind their back is, in my view, a cowardly way to handle an issue you may have someone. But seeing as so many people fear losing their jobs, it seems the easiest way to vent out their frustrations.
Chances are, you'll get caught sooner or later, and the consequences could be far more dire at that juncture. When in doubt, just stay out of it. You wouldn't want the person to do the same thing, would you?
Just do yourself a favor and steer clear of all things gossip. Not only is it dangerous, but it can pose a serious threat to your career and relationships.