How did this happen?

So, my nine year old, sweet, very compliant child recently (and by recently I mean in the last year) turned hateful, disrespectful and just down right mean. How did this happen? For the past 3 months I've been working on her attitude along with my own (every child picks something up from you, she must see right through me :). She has been improving but only because we (her father and I) have promised this child a puppy if she could change her attitude. How did this happen? Her attitude? She constantly has to deal with a twin sister, who can manipulate her way out of quick sand, a 5 year old sister who daily picks her "favorite" sister and often enough it is not Sidney, and a mother who sees her goodness and milks it for all it is worth. How did this happen?
A puppy...a Husky pup no less.....the child is obsessed with Husky's. Everything she owns is a husky (stuffed animals) or has a husky drawn on it, painted on it or whatever.
She even worked like a dog around the house for allowance to buy her own Nintendog Stuffed Husky ($19.95, mind you).
So the time is dwindling to get this puppy. She has been counting down to the very day she can get this puppy (which BTW is June 4, officially). I'm getting a little apprehensive about this event. First of all, we already have 2 dogs. We have this beautiful yard that will soon be destroyed by a puppy, and not to mention things chewed to bits. Our last puppy adventure ended up costing my husband a whole new wiring harness on our trailer.
Did I do the right thing here? Bribing my child to change who she is? Now, on my defense, she has done very well and we told her that her attitude needs to stay this way or the dog goes. She even admitted to me that she has been happier (and I must confess, things are running much more smoothly around here since this bribe). How did this happen?